Tests update FakeKeystoneClient to align user_id and trustor

When creating the dummy trust context, we return an arbitrary
hard-coded trustor_user_id, which makes testing we do things
with the trustor correctly difficult, so instead return the
user_id passed into the client (as this is what the trustor id
will be when real create_trust_context calls happen)

Change-Id: I280b820977bfc533126fe392e002210a996311f0
Merge "Tests update FakeKeystoneClient to align user_id and trustor"