A recent release of pbr explicitly requires stating which builders are
requested, rather than defaulting to html and man. This has resulted in
man pages not being built. Correct this oversight, along with a few
issues that have been introduced to 'doc/source/conf.py' since. It is
also necessary to remove a couple of plugins as there seems to be
another pbr bug under the hood. Fortunately none of these are used so
it's all good.
Change-Id: I22fd4d113704949dbf7704f50fa1d08fa10f11b6
1da56883f5