Since https://review.openstack.org/#/c/163462/ and
https://review.openstack.org/#/c/162649/4 in gnocchi, all kind of aggregations
have been moved in the same URI namespace (/v1/aggregation/...)
and we can use complex query to make aggregation across multiples metrics.
This change sync the ceilometer alarm API and Rules to match these
changes.
Change-Id: I015af585595e6880b527161fe5c8b34eeb7bfe29