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"`
This looks good, but I'd like rephrase the docstring a bit for clarity. 
"Get a list of server IDs not marked pending deletion from Nova based on the given server_name. If the group_id is given, use the server_name extracted from the launch config instead."