api/provisioner: mock responses in FindTools tests

Introduce a function for patching the FacadeCaller's
FacadeCall method, so the FindTools API calls can
be tested without an API server. This is needed to
avoid complicating the tests further when we change
over to storing tools in state.
api/base/testing: add common FacadeCaller patcher

And use it in usermanager, uniter and provisioner.
Merge pull request #664 from axw/api-provisioner-test-mocks

api/provisioner: mock responses in FindTools tests

Introduce a function for patching the FacadeCaller's
FacadeCall method, so the FindTools API calls can
be tested without an API server. This is needed to
avoid complicating the tests further when we change
over to storing tools in state.