create the ability to pass through devstack config

Instead mapping every possible devstack option up into a devstack gate
variable, we should probably just provide the ability to provide some
devstack config passthrough from the job definitions. This should be
able to reduce the number of DEVSTACK_GATE variables that we need, as
anything that maps 1 to 1 with a devstack variable should be just put
into the DEVSTACK_LOCAL_CONFIG var.

Change-Id: Ib43056ffdea4a3167b4f59d2aa6b4bfc51c221c2

devstack-vm-gate.sh

@@ -324,6 +324,13 @@ EOF
         echo "Q_HOST=$primary_node" >>"$localrc_file"
     fi
 
+    # a way to pass through arbitrary devstack config options so that
+    # we don't need to add new devstack-gate options every time we
+    # want to create a new config.
+    if [[ -n "$DEVSTACK_LOCAL_CONFIG" ]]; then
+        echo "$DEVSTACK_LOCAL_CONFIG" >>"$localrc_file"
+    fi
+
 }
 
 if [[ -n "$DEVSTACK_GATE_GRENADE" ]]; then