cluster/stripe: enable coalesce mode by default

It has been available for a while now and is probably the sane
default due to the more efficient layout and performance benefit.

BUG: 1001207
Change-Id: I6275f9741866c0afd6e685f8dc5867a86485fd20
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-on: http://review.gluster.org/5624
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>