Added a "Forbidden Files" optional parameter to Gerrit Projects [FIXED JENKINS-21232]

This forbidden file allows the project not to trigger if any forbidden
file has been impacted. This is very useful in chained scenarios. For
example, let's assume projectA upstream triggers projectB downstream.
Then if a commit affects both A and B, you may (depending on
configuration) want that A triggers and B wait for A to finish, and then
trigger B. This would be the case in particular if B uses artifacts from
A. Since A is triggering B anyways, no need for B to trigger. In that
case, this commit allows you to define the files in "A" as forbidden to
trigger a gerrit event in B. This means that such a commit affecting
both projects A and B will only trigger A. B will be built as usual as a
downstream element of A. Yet, a commit affecting only B will still
trigger a build of B. This is a very specific use case, but in this use
case, only this commit can satisfy the requirement.

Amends: broken style-checks fixed and forgotten test changes added