Cleanup to reduce Helm chart noise

[ksa-real]

• Multiple '---' and new lines could be previously produced by secrets.yaml
• Some duplication removed from _env.tpl
• telegram.webhookUrl defaults to base_url

Which issue(s) this PR fixes:

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated

[CLAassistant]

All committers have signed the CLA.

[ksa-real]

This doesn't include items in the checklist. Please review the changes first and if it is fine with you, I'll add the rest.

[ksa-real]

This original issue has been fixed by another commit. Not that my approach was to define a snippet:

{{/* Single env variable. Context: list of var_name: string, value: <map or other> */}}
{{- define "snippet.env" -}}
- name: {{ first . }}
{{- $value := index . 1 }}
{{- if kindIs "map" $value }}
  {{- $value | toYaml | nindent 2 }}
{{- else }}
  value: {{ $value | quote }}
{{- end }}
{{- end }}

And then define an environmental variable like this:

{{ include "snippet.env" (list "SLACK_CLIENT_OAUTH_ID" (default "" .Values.oncall.slack.clientId)) }}

This way a user is pretty flexible in defining a source of data for env var in values.yaml: it can be string or a reference to a config map or a secret.

I don't think it make sense to continue with my PR anymore. Feel free to copy the snippets from this comment if needed later.

[ksa-real]

I left just the cleanup part in my PR