add severity attribute to alarm（deprecated）

Currently, ceilometer alarms haven no concept of severity (or level), even there is no use cases about severity of alarm in Heat autoscalling scenario now, ceilometer should support dividing different alarms to different alarm levels. because in wider usage scenarios, users can handle different level alarm in different way.

the basic idea is:
* define options of "severity" and add attribute "severity" to alarm model;
* support "severity" in query field when retrieving alarms.