Config: use better device names for "DEVICES container"

Config: use better device names for "DEVICES container"

When "containers" appears on the "DEVICES" line (which is does by
default), use names from the mdadm map file instead of kernel names,
when possible.
This mean that the name will be more likely to appear in mdadm.conf
and so more likely to match "container=" tags.

Signed-off-by: NeilBrown <neilb@suse.de>