[JENKINS-25389] Extend "git publisher" so it will be able to push external tags.

The Git Plugin can currently only push back a previously defined tag, but not arbitrary tags created during a build.

I used the new method to push tags introduced in git-client-plugin 1.11.1 and added an option to the Jelly to push all local tags created during the build.

I wish I had a test to verify this, but I have no clue how to add a build step to a FreeStyleProject in the GitPublisherTests to create a tag.