OpenZFS 7260 - disable libdiskmgmt in zfstest unless it's required

OpenZFS 7260 - disable libdiskmgmt in zfstest unless it's required

Authored by: John Wren Kennedy <john.kennedy@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Reviewed by: Yuri Pankov <yuri.pankov@nexenta.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>
OpenZFS-issue: https://www.illumos.org/issues/7260
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/447b1e1
Closes #5794

Porting notes:
- The library libdiskmgmt is specific to illumos so these changes
  currently have no impact under Linux.  This mechanism could be
  potentially leveraged in the future.