Support GitHub Enterprise for GitHub Release Source

[crhntr]

story: TPCF-26493

[crhntr]

Some questions we had while implementing this:

• Do we need to provide a CA to the http.Client TLS config? Looks like no. We have not tested kiln from CI yet.
• Do we have API mismatches because our GitHub Enterprise deployment is different from Github.com? Both commands worked with the current go-github version so it seems to work ok.
• Do we have different api and upload URLs? Nope, it works... also we are not doing uploads in the commands so we are likely not exercising this configuration.

Known Issue
If you have multiple github release sources (ie some releases on GitHub.com and some on GitHub Enterprise), release-notes will fail to generate.

To fix this, we need to change how the GitHub client used for release note generation is configured. Instead of passing the github-token as a flag we need to properly parse release source client for each release.

[pvaramballypivot]

Did you mean to remove this print statement?

[crhntr]

Yes. My bad. Thanks for catching this.

[pvaramballypivot]

@crhntr I tested the changes locally and here's what I noticed

1. It doesn't seem to be backward compatible. The command now requires artifactory_username, artifactory_password and github_access_token to be passed in the command as variables. Previously just setting the GITHUB_TOKEN env variable was enough. This is not a huge change but it would require updating scripts.
2. The output is missing the details of the bumps. Check out this story where I have attached 2 files - "current_release_output.md" run from the current released version of kiln, and "pr_changes_output.md" run from the changes in this PR. Please compare the 2 to see more.

[crhntr]

It doesn't seem to be backward compatible. The command now requires artifactory_username, artifactory_password and github_access_token to be passed in the command as variables. Previously just setting the GITHUB_TOKEN env variable was enough. This is not a huge change but it would require updating scripts.

I had hoped to keep it backwards compatible. Unfortunately, now we need a second configuration value (the GitHub enterprise host). We could add another environment variable for that, but we already have it configured in the Kilnfile. Also with the introduction of a second GitHub host, some BOSH Releases could come from enterprise and others from open source, each host requires a different token and we use the Kilnfile.lock to resolve which credentials to use.

For local development, tile authors should have "~/.kiln/credentials.yml" configured so only automation scripts should break.