Medium: Raid1: wait for udevd to settle (bnc#821861)

Sometimes udevd does not create devices as quickly as needed.
This patch introduces a new parameter (udev). If it's set to true
(which is also the default) it will invoke udevadm settle to wait
for udevd to finish creating the MD device. We also use
--exit-if-exists not to wait unnecessarily for other events.