LV stripes parameters for plain and drbd

configure.ac:
* change the documentation of --with-lvm-stripecount parameter to
  reflect the change

doc/design-resource-model.rst:
* change drbd/stripes to drbd/data-stripes and drbd/metastripes to
  drbd/meta-stripes

rest of files:
* add the plain/stripes, drbd/data-stripes and drbd/meta-stripes disk
  parameters

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>