Extend the Vitrage REST API to support a GET of the Active Alarm Counts for each alarm severity level

Extend the Vitrage REST API to support a GET of the Active Alarm Counts, for
each alarm severity level, in Vitrage.

This is in support of the Horizon blueprint, "Vitrage Alarm Banner", which
will display the Vitrage Active Alarm Counts (by severity) in the Top
Navbar of Horizon. Vitrage Alarm Counts will be visible at all times in
horizon (regardless of what horizon screen you are on) and selecting the
Vitrage Alarm Count Banner will take you directly to the Vitrage Active Alarm
page.

This improves UX as it makes important Alarm Count data easily visible at
all times and provides a quick shortcut to the detailed Vitrage alarm list.