gluster: Reuse Command methods

The perform few cleanups: using a class level method (getDbFacade())
instead of static access to DbFacade. In addition, it adds missing
default case to switch statements.

Change-Id: I05042649259efa596c255360174127335a4d8a1b
Signed-off-by: Moti Asayag <masayag@redhat.com>