Allow adding custom worker certs that all container can inherit

[xtremerui]

Existing Issue

Fixes #https://github.com/concourse/concourse-chart/issues/211.

Changes proposed in this pull request

User will be able to config a .pem file that contains additional certs that could be added to worker certs pool by specifying a path (default to /etc/ssl/certs/worker-certs.pem). Notice the path has to be a file so it could be additional other than replacing entire /etc/ssl/certs directory.

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

Reviewer Checklist

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

[taylorsilva]

I'm taking over this PR since Rui is on leave.