Add the complete list of supported options for postgres.sslmode

[hoffmc]

Existing Issue

Why do we need this PR?

It's unclear which value needs to be set to enable SSL encryption for the postgres database connection. The most reasonable value - 'enable' - does not exist and the Concourse source code needs to be read to understand the available choices.

Changes proposed in this pull request

• Add the complete list of supported options for concourse.web.postgres.sslmode

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  • dev is for changes related to the next release of Concourse (aka unpublished code on master in concourse/concourse)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)

[xtremerui]

LGTM. Could you fix the DCO check error?