Fix wrong mock assertions in unit tests

'mock' objects don't have method "assert_called_once", but this is called in
following unit test suites:

manila/tests/network/linux/test_interface.py
manila/tests/network/neutron/test_neutron_api.py
manila/tests/test_share_generic.py
manila/tests/test_service_instance.py
manila/tests/test_share_glusterfs.py
manila/tests/test_utils.py

Make unit tests call proper assertion methods.

Change-Id: I96923bd345fc5129963c77dfce3555e7284eb836
Closes-Bug: #1330391
Merge "Fix wrong mock assertions in unit tests"