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
Merge "Add zeromq config"