only set nova catalog if it's not the default
only set nova catalog if it's not the default

This ensures we only set the nova catalog when it's not the default,
instead of also putting defaults in devstack.

Change-Id: Ibb0dcb8bae2e9223db302d7b19e8fbee4ebbf0e3

nawawi/openstack,openstack-dev/devstack,richliu/devstack,nawawi/openstack,richliu/devstack,noironetworks/devstack,group-policy/devstack,dtroyer/devstack,openstack-dev/devstack,group-policy/devstack,dtroyer/devstack,noironetworks/devstack