Combine all cinder_volume host's "cinder_backends" var to ensure all
cinder_backends are created, and there is no race condition setting up
the same backend for multiple hosts (e.g. rbd backends which exist on
multiple volume servers).
This adds a second cinder_volume node for testing, with a new backend,
as well as the old backend - this should ensure duplicate backends are
not created whilst backends are created for each cinder_volumes host.
Change-Id: I1c2fb1ef07423fc415622767ff09f058c773c1ed
Closes-Bug: 1649914
879082f204