cinder: configure backup_swift_url in subnodes
cinder: configure backup_swift_url in subnodes

The tempest-multinode-full job is running the c-bak
service on the subnode where swift isn't running, and
because of the "is_enabled_service swift" check, cinder
on the subnode wasn't getting configured to talk to
swift so the c-bak service was down. Since chances are
good that we're running swift, just configure cinder
to always use it.

Change-Id: I86b090967dadeeefc017ff0311beeea9441b6ba6
Closes-Bug: #1783128

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