Separate alarm storage models from other models

This change isolate storage models used for alarm into a separate file.

This a preparation work to have all alarms related storage API inside
the alarm namespace and then be able to have a different database for
alarming and metering.

Partial implements blueprint dedicated-alarm-database

Change-Id: Iab6b70c6a48301164016823bd0f8e9781ca21abd
Merge "Separate alarm storage models from other models"