config: support MDADM_CONF_AUTO= env var.

config: support MDADM_CONF_AUTO= env var.

If a distribution allows the choice between using mdadm and
dmraid for DDF and IMSM to be made by some config file
(/etc/defaults/ /sys/sysconfig/ etc) which is queried by
/etc/init.d scripts, then the fact that mdadm implements this
choce through the config file is not very helpful.

So allow the "AUTO" line to be specified in part using MDADM_CONF_AUTO
in environment.

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