Policy is aware of metadata disk's controller domains.

Policy is aware of metadata disk's controller domains.

Platform (metadata) domain let the metadata handlers differentiate
disk domains based on controllers that the disk belongs to.
Platform domain is sub-domain inside user specified domain
in mdadm.conf configuration files inheriting all parameters from it.
The metadata domain name is used disk domain matching functions.
The disk with the same metadata domain name belong to the same metadata
domain.

New metadata handler is added that retrieves platform domain string based
on disk path:
const char *(*get_disk_controller_domain)(const char *path);

Signed-off-by: Marcin Labun <marcin.labun@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>