Allow confirm_resize if instance is in 'deleting' status

Deleting an instance puts it into 'deleting' task_state.
If the instance is in 'resized' vm_status, this tries a
confirm_resize on the instance. confirm_resize currently expects
the task_state to be None. It should instead expect
None/deleting task_state.

Fixes bug: 1189391

Conflicts:
        nova/tests/compute/test_compute.py

Change-Id: I6a3f82eab9ed2242bfd38bc1ac760dbb4b3d8c4a
(cherry picked from commit 0fed513b695cd15aaca24d24926a1c7d13e63509)
Merge "Allow confirm_resize if instance is in 'deleting' status" into stable/grizzly