JENKINS-16299 add groups info support to reverse proxy auth

Hi !

this is a little patch to add GROUP support where using reverse proxy auth.
like for the USERNAME, you can now choose an HTTP header to pass a string containing groups separated by the delimiter of your choice.

reusing some code from script-realm-plugin, i succeeded in coding my feature request JENKINS-16299.

i'm an absolute java,jenkins&git beginer :)
so, watch out ! i may have done some mistakes... 
a carefull review of my 10 added lines is needed !

something i dont understand (but it shold be easy for you) is that when i try my new plugin (puttin the .hpi file in the plugins directory) the first time it fails beacause my 3 new configurations variables (defined in config.jelly) are empty. ?!?

thank you !

L.