Store tenant with trust details in user_creds

Currently we don't store tenant name/ID to user_creds when a trust_id
is found in the context.  However as mentioned in bug #1231483 keystone
currently requires the tenant_id when re-scoping a token to a trust via
the v2 tokens API.

Change-Id: I5bd9cb914a8c9365ed30ece8e0a61ba1e0a3c4bc
Partial-Bug: #1227901
api : default signal body to None

Default the signal body to None, so requests without a body don't fail

Change-Id: Ia42d71a338459c8b6543b3d4c5b296d005f4c9ee
Closes-Bug: #1231926
Merge "api : default signal body to None"
Merge "Store tenant with trust details in user_creds"