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 ceilometerclient alarm shell to match these
changes.
Change-Id: I5717a4604af5505dfb8c56463e2bdb86c59e41ce
(cherry picked from commit db48aa28b1)