Fix unit tests with keystoneclient master

This patch in python-keystoneclient breaks unit tests in Ironic:
https://review.openstack.org/#/c/113712/

If admin_user is not passed to keystoneclient, an exception is
raised.

This patch overrides config for admin_user in Ironic's unit tests
to get around this behavior.

Closes-Bug: 1362280
Change-Id: I3a21e7c1b3a87ebee5a0e81f805f50919ef6a6d5
Merge "Fix unit tests with keystoneclient master"