Don't allow spares when creating 'external' arrays.

Don't allow spares when creating 'external' arrays.

It is meaningless when creating the container, and for
subarrays, the container is responsible for assigning
spares.

Also, don't do the 'spare' fiddle for raid5 as we cannot
set up a spare at this point yet.  Later maybe just create
the array degraded and let the container sort it out.