imsm: fill working_disks according to metadata.

imsm: fill working_disks according to metadata.

Imsm tracks as "working_disk" each visible drive.
Assemble routine expects that the value will return count
of active member drives recorded in metadata.
As a side effect "--no-degraded" doesn't work correctly for imsm.
Align this field to others.
Added check, if the option --no-degraded is called with --scan.

Signed-off-by: Blazej Kucman <blazej.kucman@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>