Migrate to pbr

I found out pbr to be a bit nicer when it comes to edit files. AFAIK
most OpenStack project are using it, so I guess Jenkins Job Builder can
be pbr based as well.o

Basically:
* moved everything from setup.py to setup.cfg
* sorted entries while at it
* require pbr module using the same version limits used by Zuul
* enable pbr.warnerrors
* testenv.usedevelop = True or build_sphinx can not find entry points
* move requirement files to root of repository (we had them in /tools/)

Change-Id: I44ee8910ad4fe4eebb7337951efa31baff4281fe