Enable changing of rpc driver from devstack

It enables user to set flag
'MISTRAL_USE_KOMBU_DRIVER' in local.conf on their
devstack. With this flag set to 'TRUE', mistral
would autamiatically be installed with kombu driver
enabled.

This change is needed for adding new devstack gate
that would test kombu driver

Change-Id: I462aa17107a9a55396a87a8c46fa3a8dac338fac

devstack/plugin.sh

@@ -98,6 +98,10 @@ function configure_mistral {
     if [ "$LOG_COLOR" == "True" ] && [ "$SYSLOG" == "False" ]; then
         setup_colorized_logging $MISTRAL_CONF_FILE DEFAULT tenant user
     fi
+
+    if [ "$MISTRAL_RPC_IMPLEMENTATION" ]; then
+        iniset $MISTRAL_CONF_FILE DEFAULT rpc_implementation $MISTRAL_RPC_IMPLEMENTATION
+    fi
 }