Add qpid config

Although features.yaml have a feature entry for qpid, there's no
config option for qpid. So, even if you have a DEVSTACK_GATE_QPID=1
in your environment, it won't work.
This patch enable the use of DEVSTACK_GATE_QPID=1 to run devstack
with qpid instead of rabbit

Change-Id: Iffb6f5a8fef4caef7ae4e68aa261b3db63016493

features.yaml

@@ -28,6 +28,8 @@ config:
     features: [sahara]
   ironic:
     features: [ironic]
+  qpid:
+    features: [qpid]
 
 features:
   default: