Add unit tests for MServiceManager.list() method
Add unit tests for MServiceManager.list() method

The MServiceManager.list() has already supported the pagination
and sorting features, but there is no unit case to cover
them. This patch tries to add some tests for them.

Change-Id: Ia3877ecbf8c066553a3d5e8cb7cb17753471a1bf
Partial-Bug: #1559838

openstack/python-magnumclient