plugable: turn Plugin attributes into properties
    
    Implement the `name`, `doc` and `summary` Plugin attributes as properties
    to allow them to be overriden in sub-classes.
    
    Always use .doc rather than .__doc__ to access plugin documentation.
    
    Remove the mostly unused `module`, `fullname`, `bases` and `label`
    attributes.
    
    https://fedorahosted.org/freeipa/ticket/4739
    
    Reviewed-By: David Kupka <dkupka@redhat.com>