Set lock_path for Tempest to TMPDIR

Tempest change I5376f977a110f502f0e4958cbccbd379539e006b syncs
lockutils from oslo. Tempest with testr needs to use external locks
which require a lock_path config option. This commit sets that option
for tempest to use devstack's TMPDIR.

Change-Id: I464edf9e11710e1931ed4f7b0262328a57e5b3de
Merge "Set lock_path for Tempest to TMPDIR"