Exclude ec2-subnet resource from phxdevops nuke config

[arsci]

Description

• Adds ec2-subnet resource exclusion for phxdevops. We use default subnets for tests and they should not be deleted.

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.
• Attention Grunts - if this PR adds support for a new resource, ensure the nuke_sandbox and nuke_phxdevops jobs in .circleci/config.yml have been updated with appropriate exclusions (either directly in the job or via the .circleci/nuke_config.yml file) to prevent nuking IAM roles, groups, resources, etc that are important for the test accounts.

Release Notes

Added ec2-subnet resource exclusion for phxdevops

[james03160927]

LGTM