Replace UserManager with None in tests

Don't access keystoneclient's UserManager in tests. This should be
considered private data of keystoneclient and is not actually used by
horizon tests.

Change-Id: I261e79b31dfc7388adb3dc63a3a5e54042f05e54
Closes-Bug: #1347236
(cherry picked from commit 821619fff324ba1bc3f697f585a0c350573f24a2)