Switch gating tox jobs to testr parallel

This commit will migrate all gating tox jobs to use testr with the
--parallel flag. The smoke tox job is left serial because neutron
doesn't work with tempest in parallel.

Part of bp speed-up-tempest

Change-Id: I1e6eb9c7ce4d55010844ce3b6ca10dd20e7359b4
Merge "Switch gating tox jobs to testr parallel"