Since Aodh uses trust IDs stored in alarm URLs unconditionally - without
checking for tenant safety - it is not safe to allow users to pass in their own
trust IDs. Forbid this and allow only trusts created by Aodh to be used. It is
highly unlikely that there is any (legitimate) use of this feature in the wild,
since allowing Aodh to create the trust is easier anyway.
Change-Id: I8fd11a7f9fe3c0ea5f9843a89686ac06713b7851
Closes-Bug: #1649333