Assemble: don't auto-assemble if any arrays are listed in mdadm.conf

Auto-assembly and planned assembly don't really work well together,
it can be confusing.
In particular in mkinitrd or similar creates an mdadm.conf to
assemble a particular array, we shouldn't go assembling any
other arrays as well.

If you want auto assembly, you need to give mdadm a config
file with no ARRAY lines.
  mdadm -Ascpartitions
can do this.

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