The long term goal for OpenStack is to support python3 for
all things except explicit python2 testing by the T release.
Part of preparing for that is ensuring all ancillary things
like docs jobs and other tooling work with python3.
This switches the default for our docs jobs to python3 so
anything that does not specify another version will end up
using it.
Further work is necessary for the pylint job due to changes
between the runtimes. That will be done in a follow up patch.
There was also a difference in behavior with the genopts job
where it ends up trying to recreate the venv under which it
is currently running under, resulting in a corrupted venv
and a failure. This cleans up that script and changes it so
rather than a tox job calling a tox job it just runs the
script.
Change-Id: Ie79528b33070e6c60f9f5e1f2219084a2f0cb039
37510ac034