trust authentication

This patch enables authenticating by using a trust. The trust ID
must be set with the parameter --os-trust-id or the env variable
OS_TRUST_ID. Trusts are available for the identity v3 API.

Co-Authored-By: Florent Flament <florent.flament@cloudwatt.com>
Change-Id: Iacc389b203bbadda53ca31a7f5a9b8b6e1a1f522
Merge "trust authentication"