config: support "ARRAY <ignore> ..." lines in mdadm.conf

config: support "ARRAY <ignore> ..." lines in mdadm.conf

Sometimes we want to ensure particular arrays are never
assembled automatically.  This might include an array made of
devices that are shared between hosts.

To support this, allow ARRAY lines in mdadm.conf to use the word
"ignore" rather than a device name.  Arrays which match such lines
are never automatically assembled (though they can still be assembled
by explicitly giving identification information on the mdadm command
line.

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