Move sqlalchemy alarms driver code to alarm tree

This change move the alarms related code of sqlalchemy driver to
the alarm storage subtree.

Partial implements blueprint dedicated-alarm-database

Change-Id: I70047a321581ee8b12533a8a5834d26e9d203532
Remove ConnectionProxy temporary class

Since all drivers have been splitted, we can remove the temporary
ConnectionProxy class.

Partial implements blueprint dedicated-alarm-database

Change-Id: I30240173ff07bb7503a5331d13ca601b2868debc