Cleanup allocating networks when InstanceNotFound is raised

Allocated networks were not being cleaned up if InstanceNotFound was
raised after the allocation request was made.  This ensures that the
cleanup happens for this case.

Also bypasses exception logging that should not occur in this case.

Conflicts:
	nova/tests/compute/test_compute_mgr.py

NOTE(mriedem): The conflict is due to commit b1994711 which is on
master but not stable/icehouse. It's part of a blueprint so that
will not be backported.

Change-Id: Icd544752444415812714d91971e45b3ae2fb68ee
Closes-Bug: 1324934
(cherry picked from commit 5a364cb00a39b31a9d5ddb30b5cc24af54e89f4a)
Merge "Cleanup allocating networks when InstanceNotFound is raised" into stable/icehouse