Updates DevStack to match Zaqar's API name change

Zaqar is a messaging service which provides support for different
messaging patterns and messaging related semantics.
This fix changes the old API name -queuing- to a more accurate name, -messaging-.

Change-Id: I07ca6ca704a4f07d88c192a5ce1a7c626de1fcea
Merge "Updates DevStack to match Zaqar's API name change"
Merge branch 'master' of https://github.com/openstack-dev/devstack