Move toward using a backend+connection model

Instead of having a pretty restrictive module
based api for saving logbook objects it is much
more friendly and extensible to move toward a more
ceilometer-influenced engine and connection based
storage backend using stevedore to do the backend
loading instead of a custom registration/fetching
mechanism. This allows us to provide a base object
oriented backend api that can be easily inherited
from to allow for customized & pluggable backend
storage modules.

Implements blueprint stevedore-based-backends
Implements blueprint ceilometer-influenced-backends

Change-Id: Ib5868d3d9018b7aa1a3354858dcb90ca1a04055d