Correct create_trust_context docstring

The docstring in heat_keystoneclient create_trust_context incorrectly
references a test on deferred_auth_method which ended up happening in
parser.Stack, so correct it to accurately reflect the code.

Change-Id: I0d763b44a57e9b9e641e36c418445441702db07e
Add sanity check to ensure user_id can be trusted

Due to bug #1239303 the wrong user could be used when obtaining a
trust-scoped v2 token, so add a sanity check to ensure we can rely on
the user_id, using the trustor_user_id (which we currently store but
don't use) to ensure impersonation is working as expected.

Change-Id: I0481a4d96c514d0d61676d27f615c735e89ad03e
Merge "Correct create_trust_context docstring"
Merge "Add sanity check to ensure user_id can be trusted"