Fix race in test_delete_backup

Unit test sporadically failed due to timing issue - the deletion
timestamp on the backup was exactly the same as timeutils.utcnow(),
causing the AssertGreater check to fail.  Change to AssertGreaterEqual.

Change-Id: Ia66d0d4bdae3242780a91f0d384cbd17caa8c197
Closes-bug: #1284227
Merge "Fix race in test_delete_backup"