The default for install_command is good, no need to add our own version
in tox.ini. Remove it.
Remove git commands from conf.py, current openstackdocstheme handles
this itself.
Fix some missed api-wg -> api-sig renames in conf.py.
Change-Id: I806e804a5852e081f0cb8c1b43ffe13a119aba2c