The volume backup code is passing '' as a default container name,
which will cause cinder/swift to fail. If no container name is provided,
the default value of None will enable the auto creation of a container correctly.
Change-Id: I7813153c65652ce6e1a5ac03204a8738eefdc64e
Closes-Bug: #1658137
(cherry picked from commit 90a9af2e0b)