Reassign existing device-mapper maps

Reassign existing device-mapper maps

When a multipath device is created other maps might already be
in place pointing to the same block device. To ensure uninterrupted
access these maps should be reassigned to point to the
multipath devices instead.
This patch also adds a configuration variable 'reassign_maps'
to toggle this behaviour.

Signed-off-by: Hannes Reinecke <hare@suse.de>