Illumos 5161 - add tunable for number of metaslabs per vdev

5161 add tunable for number of metaslabs per vdev
Reviewed by: Alex Reece alex.reece@delphix.com\
Reviewed by: George Wilson george.wilson@delphix.com\
Reviewed by: Christopher Siden christopher.siden@delphix.com\
Reviewed by: Paul Dagnelie paul.dagnelie@delphix.com\

Original author: Matthew Ahrens

References:
  https://www.illumos.org/issues/5161
  https://reviews.csiden.org/r/95/

Ported by: Turbo Fredriksson <turbo@bayour.com>