Fix using XCom with KubernetesPodOperator

[kaxil]

This commit effectively reverts #15942 because of the issues it is causing. Until we find better solution we should revert this change

Error:

[2021-07-26 20:23:54,109] {taskinstance.py:1108} INFO - Executing <Task(KubernetesPodOperator): write-xcom> on 2021-07-26T20:23:27.058907+00:00
[2021-07-26 20:23:54,113] {standard_task_runner.py:52} INFO - Started process 11 to run task
[2021-07-26 20:23:54,297] {standard_task_runner.py:76} INFO - Running: ['airflow', 'tasks', 'run', 'k8_pod_operator_xcom', 'write-xcom', '2021-07-26T20:23:27.058907+00:00', '--job-id', '1757', '--pool', 'default_pool', '--raw', '--subdir', 'DAGS_FOLDER/k8s_xcom_example.py', '--cfg-path', '/tmp/tmp0q94pkhs', '--error-file', '/tmp/tmpoz9qqp2l']
[2021-07-26 20:23:54,298] {standard_task_runner.py:77} INFO - Job 1757: Subtask write-xcom
[2021-07-26 20:23:54,511] {logging_mixin.py:104} INFO - Running <TaskInstance: k8_pod_operator_xcom.write-xcom 2021-07-26T20:23:27.058907+00:00 [running]> on host k8podoperatorxcomwritexcom.21384021df914227ad4e4b3a34313710
[2021-07-26 20:23:54,713] {taskinstance.py:1502} ERROR - Task failed with exception
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/airflow/models/taskinstance.py", line 1158, in _run_raw_task
    self._prepare_and_execute_task_with_callbacks(context, task)
  File "/usr/local/lib/python3.7/site-packages/airflow/models/taskinstance.py", line 1295, in _prepare_and_execute_task_with_callbacks
    self.render_templates(context=context)
  File "/usr/local/lib/python3.7/site-packages/airflow/models/taskinstance.py", line 1796, in render_templates
    self.task.render_template_fields(context)
  File "/usr/local/lib/python3.7/site-packages/airflow/models/baseoperator.py", line 999, in render_template_fields
    self._do_render_template_fields(self, self.template_fields, context, jinja_env, set())
  File "/usr/local/lib/python3.7/site-packages/airflow/models/baseoperator.py", line 1012, in _do_render_template_fields
    rendered_content = self.render_template(content, context, jinja_env, seen_oids)
  File "/usr/local/lib/python3.7/site-packages/airflow/models/baseoperator.py", line 1063, in render_template
    return [self.render_template(element, context, jinja_env) for element in content]
  File "/usr/local/lib/python3.7/site-packages/airflow/models/baseoperator.py", line 1063, in <listcomp>
    return [self.render_template(element, context, jinja_env) for element in content]
  File "/usr/local/lib/python3.7/site-packages/airflow/models/baseoperator.py", line 1047, in render_template
    return jinja_env.get_template(content).render(**context)
  File "/usr/local/lib/python3.7/site-packages/jinja2/environment.py", line 883, in get_template
    return self._load_template(name, self.make_globals(globals))
  File "/usr/local/lib/python3.7/site-packages/jinja2/environment.py", line 857, in _load_template
    template = self.loader.load(self, name, globals)
  File "/usr/local/lib/python3.7/site-packages/jinja2/loaders.py", line 115, in load
    source, filename, uptodate = self.get_source(environment, name)
  File "/usr/local/lib/python3.7/site-packages/jinja2/loaders.py", line 197, in get_source
    raise TemplateNotFound(template)
jinja2.exceptions.TemplateNotFound: mkdir -p /airflow/xcom/;echo '[1,2,3,4]' > /airflow/xcom/return.json
[2021-07-26 20:23:54,797] {taskinstance.py:1552} INFO - Marking task as FAILED. dag_id=k8_pod_operator_xcom, task_id=write-xcom, execution_date=20210726T202327, start_date=20210726T202353, end_date=20210726T202354
[2021-07-26 20:23:54,936] {local_task_job.py:153} INFO - Task exited with return code 1

Dag:

from airflow import DAG
from airflow.operators.bash import BashOperator
from airflow.providers.cncf.kubernetes.operators.kubernetes_pod import (
    KubernetesPodOperator,
)
from airflow.utils.dates import days_ago
from airflow.configuration import conf

namespace = conf.get("kubernetes", "NAMESPACE")

# This will detect the default namespace locally and read the
# environment namespace when deployed to Astronomer.
if namespace == "default":
    config_file = "/usr/local/airflow/include/.kube/config"
    in_cluster = False
else:
    in_cluster = True
    config_file = None

default_args = {
    "owner": "airflow",
}

with DAG(
    dag_id="k8_pod_operator_xcom",
    default_args=default_args,
    schedule_interval=None,
    start_date=days_ago(2),
    tags=["k8"],
) as dag:

    write_xcom = KubernetesPodOperator(
        namespace=namespace,
        in_cluster=in_cluster,
        config_file=config_file,
        image="ubuntu",
        cmds=[
            "sh",
            "-c",
            "mkdir -p /airflow/xcom/;echo '[1,2,3,4]' > /airflow/xcom/return.json",
        ],
        name="write-xcom",
        do_xcom_push=True,
        is_delete_operator_pod=True,
        task_id="write-xcom",
        get_logs=True,
    )

    pod_task_xcom_result = BashOperator(
        bash_command="echo \"{{ task_instance.xcom_pull('write-xcom')[0] }}\"",
        task_id="pod_task_xcom_result",
    )

    write_xcom >> pod_task_xcom_result

closes #17186

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[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.