Fix an issue with the 'resource_backend' included in the scheduler spec
for creating a volume associated with another volume, snapshot, or
group/cg. When running A/A, the 'resource_backend' must reference the
cluster, not the host.
Enhance the unit tests that cover this area. This includes fixing the
'expected_spec' so it copies a dictionary rather than referencing it,
so that external changes to the dictionary don't inadvertently update
the unit test's expected results.
Closes-Bug: #1808343
Change-Id: I7d414844d094945b55a094a8426687595f22de28
541168b86e