Don't access the net while building docs

(Note this hasn't been submitted upstream)

Change-Id: I42c6e3a5062db209a0abe00cebc04d383c79cbcb
(cherry picked from commit f2b4bb4e45afcc178200966193a7b87401c534d7)
Remove runtime dep on python pbr
Revert use oslo.sphinx and remove local copy of doc