Fix concurrent policy issue in unittest

After merge of following commit: 4f17b8d837ebad4ef2e8ae5fe95ea968c7b8b587
appears unstable error when we get denial from policy check.
'Fix' mocks policy.check_policy method and tests that it was called.
We do not test policy actions in related unittest suite located in
'manila.tests.api.v1.test_share_networks' module, and should not.

Change-Id: I8885fa622e603c2e1882709d427e2fd1b532197c
Closes-Bug: #1362468