Keystone API v2 support

Mistral should be able to work with Keystone API v2 authentication. One problem we have is trusts. So most likely Mistral functionality will be limited if we enable v2 authentication (cron triggers). But at the same time many people have this requirement and we need to think how it could be implemented.

As a simple solution for deferred calls (like cron-triggers) we could suggest for now just to authenticate using server Mistral keystone credentials even though it's obviously a security problem.