LVM agents: Retry VG shutdown to cope with udev collisions

When trying to shutdown a VG, a failure can occur due to udev
opening an LV.  A error with a message similar to the following
can occur:
  lvm[25454]: Can't deactivate volume group "vg1" with 1 open logical volume(s)
Allowing udev to settle and trying the operation again helps to
resolve the issue.

Helps to resolve rhbz729812.