Move documentation dependencies to doc/requirements.txt.
Update tox.ini to run the documentation build the same way the zuul
job does.
Switch from oslosphinx, which is deprecated, to openstackdocstheme.
Change-Id: I6ec358fecefcc6f88f498cf12cd45b77989f5c26
Signed-off-by: Doug Hellmann <doug@doughellmann.com>
ef985c01c3