Introduce a serializer

When the engine is started, start a serializer thread. The crontab
for this is picked up from the engine cfg. The serializer maintains
a queue of upcoming audits, and the engine should pick off audits as
their scheduled time comes up. This will save us from running one
thread per registered audit script, having just as many threads as
there are currently scheculed audits.

Repair scripts stay the way they are.

implements blueprint backend-abstraction
Change-Id: If17477f9939c69b454ba389f2f0f8cce47b94cca