Add support for disabling service links

[mreiche]

Existing Issue

• No issue filed

Why do we need this PR?

In some Kubernetes environments and namespaces, the concourse pods get environment variables that cannot be parsed by the entrypoint. This will result in Pod startup failures like:

error: invalid argument for flag `--postgres-port' (expected uint16): strconv.ParseUint: parsing "tcp://172.30.163.53:5432": invalid syntax

This happes in the Concourse chart, when you don't use the built-in PostgreSQL chart, but another one like:

postgresql:
  enabled: false

In this case, Kubernetes provides "invalid" environment variables.

Changes proposed in this pull request

This change allows administrators to disable service links environment variables in workload pods by setting:

enableServiceLinks: false

Contributor Checklist

• Variables are documented in the README.md
• Which branch are you merging into?
  • master is for changes related to the current release of the concourse/concourse:latest image and should be good to publish immediately
  • dev is for changes related to the next release of Concourse (aka unpublished code on master in concourse/concourse)

Reviewer Checklist

This section is intended for the core maintainers only, to track review progress. Please do not
fill out this section.

• Code reviewed
• Topgun tests run
• Back-port if needed
• Is the correct branch targeted? (master or dev)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: mreiche / name: Mike Reiche (ed085d7)

[taylorsilva]

Thanks for the PR! Will review sometime this week