Fixing tests to work with keystoneclient 0.6.0

Another item needs mocking with the new release of
python_keystoneclient. Adding it to mocked keystoneclient.Client
object.

Closes-Bug: #1279907
Change-Id: I3c43a7290a6e05c718082dcad369162bf21e3603
(cherry picked from commit 043085f812a1d06e8e082b2dbd52f406172bbfff)
Merge "Fixing tests to work with keystoneclient 0.6.0" into stable/havana