We run tempest inside of tox so no nee to install it. By not installing
it we decouple one more thing from the requirements sync.
Without this patch, due to branchless tempest, tempest must work with
master and all stable branch global requirements.
Although installing tempest should work on master, don't install it
anyway to make the user experience more uniform across master and
stable branches.
Note: Long term we can install this inside of a virtualenv
(I92648fffc1ad6af53006a0970722fd15f4e79dc2) but that logic hasn't landed
yet and installing tempest is breaking us. So leave moving this to a
virtualenv for a later patch.
Change-Id: I78d51f04ed01da4ce8aa0e127be028f969d3b4f8
1368b98669