Ease writing of new on-disk Binding implementations (take 2)

[thomasgl-orange]

This PR is the successor of #25 (which was merged and then rolled-back, and thus can't be updated anymore). The goal is still to ease writing new credentials binding implementations which write to temp files without copy/pasting what exists already in FileBinder.

The code from #25 was reviewed already by @jglick, and it was fine, expect it was specific to writing Binding implementation, and was not supporting MultiBinding at all. Meaning it wasn't helping in cases like the SSH key binding proposed in #18 for instance.

This new PR includes two commits:

• 83ba9e2 are the changes already reviewed in PR Ease writing of new on-disk Binding implementations #25, simply rebased on master
• 8c50bd3 is a refactoring to make some utility code also available to MultiBinding implementations. The AbstractOnDiskBinding API is unchanged (meaning credentials-binding implementation for Docker client certificates docker-commons-plugin#54 should still be ok), but the code for managing the temp dirwas moved to the new UnbindableDir utility class.

Here is an example of using this new API to simplify a MultiBinding implementation (the SSH keys one from #18): thomasgl-orange/credentials-binding-plugin@a4423f3

[jglick]

Looks right from a brief review. Are there downstream PRs showing this being used for SSH and/or certificate and/or Docker credentials?

[jglick]

Deployed these changes as version 1.12-20170613.180829-1 which would allow them to be consumed from a PR in another plugin, compatible with CI. For a potential PR in this plugin, you would simply merge this branch.

[thomasgl-orange]

Are there downstream PRs showing this being used for SSH and/or certificate and/or Docker credentials?

I have submitted #38 to trigger a CI build of a modified version of the SSH Credentials binding implementation proposed by @matthauck in #18.

I have also updated jenkinsci/docker-commons-plugin#54.

[thomasgl-orange]

I've now also submitted #39 to trigger a CI build of a modified version of the Certificate binding implementation proposed by @idstein in #11.