Fix race for stable/grizzly backport of test_reclaim_queued_deletes

Commit c17e0c1d backported a race condition fix from Havana to
stable/grizzly but the test case added to test_compute doesn't account
for needing to stub out the instance_get_all_by_host call to conductor,
which can lead to race failures on stable/grizzly.

When comparing test_compute.py on stable/grizzly and stable/havana,
'instance_get_all_by_host' is stubbed out extensively in grizzly but not
Havana. Note that in Havana commit dce64683 changed the delete/soft_delete
paths use objects, so a lot of the conductor API stubs from test_compute
were removed in Havana due to that change. Given that Grizzly is not
using objects, and we won't backport commit dce64683 to Grizzly, it makes
sense to just stub out the conductor API DB calls in this patch.

Closes-Bug: #1284666

Change-Id: Ib806e0f3136e5fbef1141dcb65580524f9eb3899
Merge "Fix race for stable/grizzly backport of test_reclaim_queued_deletes" into stable/grizzly