Add more descriptive assertion message for test_create_backup

In order to fingerprint the failure for gate race bug 1267326 in
elastic-recheck, the assertion message needs to be specific to the test
case that's failing. We have to do something like this until we have
multi-line query support in elastic-recheck.

Also makes the backup names more specific.

Related-Bug: #1267326

Change-Id: I4cf4a3409eedf64f928ae7b0ff4a660c116b89bc
Merge "Add more descriptive assertion message for test_create_backup"