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
diff --git a/lib/tempest b/lib/tempest
index b4a579b..62da1fc 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -195,6 +195,9 @@
         fi
     fi
 
+    # Oslo
+    iniset $TEMPEST_CONF DEFAULT lock_path $TMPDIR
+
     # Timeouts
     iniset $TEMPEST_CONF compute build_timeout $BUILD_TIMEOUT
     iniset $TEMPEST_CONF volume build_timeout $BUILD_TIMEOUT