Set TEMPEST_CONCURRENCY=1 for Ironic

The virutal ironic devstack job runs without tenant isolation
and needs to run Tempest tests serially.  d-s-g typically does this
automatically when isolation is disabled but it must be specified
explictily in this case since we're also using TEMPEST_REGEX.

Also cleans out some unused / unneeded environment variables.

Change-Id: Ic8db89907ca85369c2b2ffd20eced8538f3fd1ef
Merge "Set TEMPEST_CONCURRENCY=1 for Ironic"