Add new Ceilometer combination alarm resource

This patch introduces a new OS::Ceilometer::CombinationAlarm resource
allowing the creation of combination alarms in Ceilometer.

Implements: blueprint ceilometer-combination-alarms
Change-Id: I8d7ba7f210423a5e7a1b1f1b86007fabdade7482
Merge "Add new Ceilometer combination alarm resource"