Replace UserManager with None in tests

This patch is a port of a recent Icehouse/Havana patch that unblocks most of the horizon tests (a problem caused by recent changes to keystoneclient, which the testing suite pulls in to run tests).

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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)
(cherry picked from commit df782fc2b51f5e9d2e38e3e33a4ebdc0705a50b4)