ipalib: introduce Principal parameter
    
    This patch introduces a separate Principal parameter that allows the framework
    to syntactically validate incoming/outcoming principals by using a single
    shared codebase.
    
    https://fedorahosted.org/freeipa/ticket/3864
    
    Reviewed-By: David Kupka <dkupka@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>