Alarm API update

This updates the alarm API to match the latest discussion:

https://wiki.openstack.org/wiki/Ceilometer/blueprints/alarm-api

It allows creation of different kinds of alarm.
The current kind of alarm has been named 'threshold'.

It move the defaults values from the storage models to the API
with all tools provided by wsme to ensure mandatory field and default.

A behavior change, it is now mandatory to PUT a full alarm description
in a PUT call.
In the future a new endpoint can be added to allow to modify only one
field (example for state: /v2/alarms/<id>/state)

Implements blueprint alarming-logical-combination

Change-Id: Ib85636728d427cdb70ef530ff9ff20d2b75c5ed1