patch retry to stop mdadm and pause udev

hi:
   the patch is related to issue https://github.com/rear/rear/issues/508 
   I was testing RHEL7 these days. and I found the mdadm layout structure  is changed in RHEL7. files belongs to  /dev/md at RHEL6 now move to /run/mdadm. /dev/md may exists for mdadm linked names or maybe there is no /dev/md directory.  so I modify the  software raid detecting/stopping code and hope they are more general.

  and I realized one thing: the code for stopping mdadm and pause udev is just a "helpler", which help coming command "parted" and "partprobe" to success. so the fail of these "helper" code should not be fatal. if these "helper" code failed but "parted" and "partprobe" still success, then everything is fine. so I modify the code to only show warning message if failed. I think "warning but not fatal" also improves compatibility.  if you still have the fedora 21 beta environment which can make "udevadm" return none zero, maybe you can give it a try.