Check quantum status dependent on the plugin

There are Quantum plugins that do not require the use of an agent.
This patch relaxes the constraint in quantum-adv-test.sh by adding
the ability to customize the logic that verifies whether the test
can run or not, depending on the Quantum plugin of your choice.

Fixes bug #1177904

Change-Id: I6e2c17f43d9e1475b2eb175cceb5107a83f9aa74
diff --git a/lib/quantum_plugins/plumgrid b/lib/quantum_plugins/plumgrid
index 912aa7e..1456710 100644
--- a/lib/quantum_plugins/plumgrid
+++ b/lib/quantum_plugins/plumgrid
@@ -35,5 +35,8 @@
     :
 }
 
+function quantum_plugin_check_adv_test_requirements() {
+    is_service_enabled q-agt && is_service_enabled q-dhcp && return 0
+}
 # Restore xtrace
 $MY_XTRACE