Switch to python-keystoneclient 0.2.3.

Fixed the bug #1151665 by having python-keystoneclient dependency to
version >=0.2.3.

Also fixed the issue of setting the fake memory cache for keystone
middleware in unit test by using the correct configuration and request
environment.

Change-Id: Id3706bea6083c03adc46a8716c5cb8074339fc45