test_manager_offline: mock requests.Session (#1316)
test_manager_offline: mock requests.Session (#1316)

Ever since cloudify-cosmo/cloudify-common#795 we don't use raw
requests.get anymore, but create an explicit requests.Session.
So the mock that mocked requests.get, needs to be updated.
cloudify-cosmo/cloudify-cli,cloudify-cosmo/cloudify-cli