ccdconfig: reference gpart rather than fdisk and disklabel

fdisk and disklabel are legacy tools for legacy disk partitioning
schemes.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43564

(cherry picked from commit 8669e516f0e6f48a5017db9e5f4768275f1deaa8)