Instead of reimplementing the slave property in bond, bridge
and team, just add the property to the parent class. It's not
that the parent class would be agnostic to the master/slave
implementation, all the slaves are known to the every device
type implementation.

Also, the derived class doesn't know the correct time when
to invoke the notify-changed for the slaves property.
E.g. it should be only invoked after nm_device_slave_notify_enslave()
when other components also consider the slave as enslaved.
Later this will be fixed so that the SLAVES property correspond
to what other master/slave related properties say.