Refactor threshold evaluator

This change creates a base class for evaluator.

The alarm service load all evaluators.
A evaluator needs to implement abstract method of this base class.

The alarm service now check in alarm.type use the right extension to
evaluate the alarm.

The previous threshold evaluator code has been moved into the threshold
extension.

Related to blueprint alarming-logical-combination

Change-Id: If6057b7db1e894333e6e9f1edb41ab75bc2c4444