Checking deleting task_state in cloudcafe

Many of our prod tests fail because nova doesn't completely delete servers within 15 mins even though otter issued delete. Otter has recently started checking if `OS-EXT-STS:task_state` is `deleting` after deleting server. If so, it assumes that nova will eventually delete it. This PR does the same in tests. This should reduce tests that fail with following message: `"Servers on the tenant with name test_sg_bhv_srv410275 were not deleted even after waiting 15 mins"`
Looks good, but the docstring should be updated; it explicitly mentions the old method being called. (That's probably a bad thing, because it's an implementation detail, but something to the tune of "servers that are currently up and not in the process of being deleted").