Add tests for keystone OS-TRUST v3 API

Add tests providing coverage of the keystone v3 OS-TRUSTS extension,
as documented at:
https://github.com/openstack/identity-api/blob/master/\
openstack-identity-api/v3/src/markdown/identity-api-v3-os-trust-ext.md

This covers all the documented interfaces to create/delete/list/get
trusts and roles delegated by trusts, except those impacted by bug
1246383 and 1245590, which will be submitted separately.

A subsequent patch will add coverage for consuming trusts via the tokens API.

Note only the JSON API is currently tested due to bug #1246941

Partially-Implements: blueprint keystone-trust-api
Change-Id: I310951f070107f3af33b52fcd2c8e54d82654088
Merge "Add tests for keystone OS-TRUST v3 API"