Set --serial as default in run-tempest.sh

Currently this is set in gate-check-commit, but this is a sensible
default for situations other than gate-check-commit so it should be set
in run-tempest itself.

Change-Id: I0bef1c5650757d54dd4fbeb6b872bd00cabed627
Closes-bug: #1530796

scripts/gate-check-commit.sh

@@ -19,7 +19,6 @@ set -e -u -x
 ## Variables -----------------------------------------------------------------
 export MAX_RETRIES=${MAX_RETRIES:-"2"}
 # tempest and testr options, default is to run tempest in serial
-export RUN_TEMPEST_OPTS=${RUN_TEMPEST_OPTS:-'--serial'}
 export TESTR_OPTS=${TESTR_OPTS:-''}
 # Disable the python output buffering so that jenkins gets the output properly
 export PYTHONUNBUFFERED=1

scripts/run-tempest.sh

@@ -23,7 +23,7 @@ export TEMPEST_SCRIPT_PATH=${TEMPEST_SCRIPT_PATH:-/opt/openstack_tempest_gate.sh
 ## is able to dig into tempest/the updated/deprecated config(s). This test should
 ## go back to being the scenario tests.
 export TEMPEST_SCRIPT_PARAMETERS=${TEMPEST_SCRIPT_PARAMETERS:-"scenario heat_api cinder_backup"}
-export RUN_TEMPEST_OPTS=${RUN_TEMPEST_OPTS:-''}
+export RUN_TEMPEST_OPTS=${RUN_TEMPEST_OPTS:-'--serial'}
 export TESTR_OPTS=${TESTR_OPTS:-''}
 
 ## Library Check -------------------------------------------------------------