Adding management_url to test mock client

Mocking management_url property in keystoneclient.Client for mox.

Closes-Bug: #1268631
(cherry picked from commit f7d00b5960c4e7f878a647e6c83ead986d55f921)

Change-Id: I3e1067aa4bc6cd5d2875433197d7bea69a16195b