Avoid leaking admin-ness into combination alarms

Previously when an admin created a combination alarm on
behalf of an non-admin identity, this had the effect of leaking
visibility onto alarms that would not normally
be visible to the non-admin tenant.

Now we validate all alarm ids with the project ID of the non-admin
identity that will ultimately own the alarm instead of the project ID
of the API caller.

Fixes bug #1237632

Change-Id: I5d1cf41c9182f09bc37b93deb14dda58f1d6dcd6
Merge "Avoid leaking admin-ness into combination alarms"