Fix kwarg passed twice error in EngineFacade.from_config()

slave_connection keyword argument is passed to EngineFacade init
twice: the first time in code explicitly, and the second time when
CONF.database options are unpacked. Actually, it's better to pass
all the arguments explicitly rather than rely on the fact that
argument names correspond to database options names (now they are
tightly coupled).

Closes-Bug: #1330426

Change-Id: Ic0aaf26e76f68adc48f7ed58fd4c7e9e87648005