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>