Refactor persister to handle multiple storage engines.

Refactor persister to handle multiple storage engines.
Allow persister to load and save configuration snapshots from/to multiple storage instances.
Loading works by iterating list of storages backwards and pushing first non-empty response to netconf.
This allows having a default (initial) configuration for the controller that will never be overwrittern.
Saving configuration will be propagated to all storage engines except those configured as read only.

Change-Id: If4cdbb9e0c303d5ebb2a3d04a316c74ff76dfb91
Signed-off-by: Tomas Olvecky <tolvecky@cisco.com>