plugable: switch API to Registry-based plugin discovery
Merge Registrar into Registry. Use the Registry instance of each plugin
module to discover plugins in the module instead of the global Registrar
instance.
This removes the side-effect of all plugins in a module being re-registered
every time the module is imported.
https://fedorahosted.org/freeipa/ticket/4739
Reviewed-By: David Kupka <dkupka@redhat.com>