Fix invalid uuid warnings in backup unit tests

Fix invalid uuid warnings in backup unit tests

Currently, 'tox -epy27 -- --regex cinder.tests.unit.backup' and
'tox -epy27 -- --regex cinder.tests.unit.test_backup' emit
188 FutureWarnings from oslo.versionedobjects aout invalid uuids
as documented here[1].

This commit changes these tests to use valid uuids so that when they
are run these warnings are no longer emitted.

[1] http://docs.openstack.org/developer/oslo.versionedobjects/api/fields.html#oslo_versionedobjects.fields.UUIDField

Change-Id: I356a38791de06164ec634c463713df9000268194