Add support for random string parameter plugin

[harcher81]

No description provided.

[cloudbees-pull-request-builder]

plugins » rebuild-plugin #36 SUCCESS
This pull request looks good

[oleg-nenashev]

@rsandell
Robert, just for the future...
Rebuild Plugin provides the extension point, which allows users to define rebuild configurations for parameters in their plugins. It makes sense to advertise such approach instead of direct patches like this PR (just to keep the code consistent with explicit plugin dependencies).

[rsandell]

aha, a feature that I've missed. apparently I haven't stayed fully up to date with the features, so I just assumed it was the usual approach.
I'll keep that in mind for the future.

[oleg-nenashev]

This is quite new feature (thanks to @ikedam).
IMHO, it's great to know that someone else works on such kernel plugins :)