Add zeromq config

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

Change-Id: I81075fe23bc9a751c05a3f777f7409d0104c979e

features.yaml

@@ -30,6 +30,8 @@ config:
     features: [ironic]
   qpid:
     features: [qpid]
+  zeromq:
+    features: [zeromq]
 
 features:
   default: