Clear the logic in backup manager

Make the system fail fast instead of retrying to do a backup.
Returning error code bazooka can try to recover from failure and start
again doozer backup.

* lookup only once for ent service
* make all struct and methods private
* add timeout to ent client
* save the snapshot in buffer not in file
* exit with error code on failure
* add tests
* export the default values as constants
* remove unnecessary retrying