9194 mechanism to override ashift at pool creation time

illumos/illumos-gate@93a1902e519bf492c071b48ffb93e5c1c7b53fb9

Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Allan Jude <allanjude@freebsd.org>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Reviewed by: Richard Elling <Richard.Elling@RichardElling.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>