Set tempest lock_path to default in $DATA_DIR

Change I464edf9e11710e1931ed4f7b0262328a57e5b3de set the tempest
lock_path to be $TMPDIR, however when run in the gate this is an
empty string which results in all tests that use locking failing.
This commit corrects that by setting the lock_path config variable
to use $TEMPEST_STATE_PATH which defaults to $DATA_DIR/tempest.

Change-Id: I02fbd50ca68d6daafb5b4c23579473eb703ae72a
Merge "Set tempest lock_path to default in $DATA_DIR"