Add some tests for the error path(s) in RBD cleanup_volumes()
Add some tests for the error path(s) in RBD cleanup_volumes()

This code depends on a module which is not always present, and thus is
a little tricky to test, given exceptions missing from that module. This
patch adds a few tests to validate that error path.

Conflicts:
	nova/tests/unit/virt/libvirt/test_rbd.py

Change-Id: I03e9376d956c70ced60e5bee32b4598f7b34d0d5
(cherry picked from commit 3ce8e8f90be67fb49efd0c60e2a4f5ae9da503c5)

tianweizhang/nova,tianweizhang/nova,redhat-openstack/nova,redhat-openstack/nova