ceph-disk: call partprobe outside of the prepare lock; drop udevadm settle

ceph-disk: call partprobe outside of the prepare lock; drop udevadm settle

After we change the final partition type, sgdisk may or may not trigger a
udev event, depending on how well udev is behaving (it varies between
distros, it seems).  The old code would often settle and wait for udev to
activate the device, and then partprobe would uselessly fail because it
was already mounted.

Call partprobe only at the very end, after prepare is done.  This ensures
that if partprobe calls udevadm settle (which is sometimes does) we do not
get stuck.

Drop the udevadm settle.  I'm not sure what this accomplishes; take it out,
at least until we determine we need it.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 8b3b59e01432090f7ae774e971862316203ade68)