New approach to mark bkr as a namespace package

* Tells setuptools to install the bkr/__init__.py file
* Adds the namespace package metadata in "make install"
* Allows building the docs with system RPMs installed

Bug: 957577
Change-Id: I72b644051bd44d96baee7fca165df9a4aabeff25
Merge "New approach to mark bkr as a namespace package" into develop