imsm: support 'missing' devices at Create

imsm: support 'missing' devices at Create

Specifying missing devices at create is very useful for array recovery.

For imsm create dummy disk entries at init_super_imsm time, and then use
them to fill in unoccupied slots in the final array (if the container is
unpopulated).

If the container is already populated (has a subarray)
'missing' disks must be in reference to already recorded missing devices
in the metadata.

Also add support for --assume-clean for imsm arrays.

Cc: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>