Use setuptools to install contrib plugins

This change creates a pbr setup.cfg and specifies a data_files
entry to install the contrib plugin source into the /usr/lib/heat
plugin directory.

This change also temporarily disables docs building for contrib
resoures until the transition to stevedore is fully complete.

Change-Id: I1c91aee20f72dc2a5a049e67de1d6d7cbabda241