Extend Global JJB maven to support subdirectories consistently. Judging from the git commit log and tag we are using Global JJB at commit b77eea9b53de9cfca7054eab1a2c32190ce38c4d (HEAD, tag: v0.29.0)

Some Global JJB job templates support building in a subdirectory of a git repository (gerrit project), apparently the verify and merge jobs work fine.  Eric Ball pointed out that the javadoc template does NOT support this.

This arose in the context of acumos which has some git repositories that contain multiple independent features. The project uses the "-f subdir" parameter to mvn which is supposed to compile/package/test just the relevant parts.  Please see this related help ticket:

https://rt.linuxfoundation.org/SelfService/Display.html?id=69791

Hope this was clear, please let me know if I should add more details, thanks for considering it.