Authenticated alarm notifications

We'd like alarm actions to be able to be authentified. The scenario is using ceilometer to trigger autoscaling actions.

One idea is to have a ceilometer service user, and creating trusts for it when creating the alarm. The ceilometer user can then impersonate the user when talking back to heat.

We'd need specific URLs encoding the necessary information, which is one full URL and one user to trust:

trust+http://user@host/path/ would talk to "http://host/path" impersonating "user".