Add Alarm Silencing Functionality

Alarm silencing is to add the ability to mute an alarm for some given amount of time. A silence rule would be configured based on a given alarm attribute, a start time, and an end time. This will be configurable via the API using a new API resource (see alarm manager API resource blueprint for more details). Before any notification is sent out, the alarm would be checked against any silence rules. If the current time falls in the silence window, then a notification would not be sent

The reason for adding this feature is to enable the user to silence a potentially annoying alarm for an amount of time while the root cause of the alarm is investigated and/or fixed.

The needed changes for this include changes to the API, the python-monascaclient, and the Notification engine. For API changes, please see the alarm manager API resources blueprint. The Notification engine will need to be modified to check an alarm against existing silence rules. The Notification engine will start by checking an alarm against any silence, inhibition, and grouping rules. If an alarm falls inside the configured start and end time window, then no notification is sent out. Otherwise, a notification is sent. Then an alarm needs to go through any inhibition and grouping checks so it is sent to a Kafka topic for further analysis. The Alarm Manager and the Notification Engine will read rules on start up from the database, and in run-time from a Kafka topic called Alarm Manager Rules and keep the rules in memory. Please see the flow diagram for more information.

Please see examples in Monasca wiki page: https://wiki.openstack.org/wiki/Monasca#Alarm_Managers