multi-backend: configure default type

This patch aims to
- configure a default type in cinder-volume (without it, a volume can't
  boot without explicitly specifying a type from the client, which could
lead to errors)
- add unit tests for cinder::backends class

Change-Id: Ia0a475a629576596dd60e513bf0764e9bc3000ab
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>