plugable: support plugin versioning
    
    Allow multiple incompatible versions of a plugin using the same name. The
    current plugins are assumed to be version '1'.
    
    The unique identifier of plugins was changed from plugin name to plugin
    name and version. By default, the highest version available at build time
    is used. If the plugin is an unknown remote plugin, version of '1' is used
    by default.
    
    https://fedorahosted.org/freeipa/ticket/4427
    
    Reviewed-By: David Kupka <dkupka@redhat.com>