add default value to functional test environment variables

AODH_TEST_BACKEND and AODH_SERVICE_URL are required to run functional
test. But it is a burden to export those two environment variables for
default devstack environment, this patch sets AODH_TEST_BACKEND=mysql
and AODH_SERVICE_URL=http://127.0.0.1:8042 if they are not set.

Change-Id: I5e977595dbbb0a01981f9317135ef1eb56602d7e

run-functional-tests.sh

@@ -1,6 +1,9 @@
 #!/bin/bash -x
 set -e
 
+export AODH_TEST_BACKEND=${AODH_TEST_BACKEND:-mysql}
+export AODH_SERVICE_URL=${AODH_SERVICE_URL:-http://127.0.0.1:8042}
+
 case $AODH_TEST_BACKEND in
     hbase)
         export AODH_TEST_STORAGE_URL="hbase://__test__"