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.

Change-Id: Icd544752444415812714d91971e45b3ae2fb68ee
Closes-Bug: 1324934
Merge "Cleanup allocating networks when InstanceNotFound is raised"