Alarm: Use stevedore to load the service class

A deployer must choice a driver with a name not a python class

This change does that but also keep backward compatibility.

DocImpact
Closes-Bug: #1362006

Change-Id: I41183b1a0c0f07c15efd6e0704a7a281b8ec7284
Merge "Alarm: Use stevedore to load the service class"