qa: rbd map-snapshot-io: udevadm settle

Udev runs blkid on device close, thwarting any rbd unmap that
immediately follows use of the device.  Explicitly settle for now.

See #4183.

Signed-off-by: Sage Weil <sage@inktank.com>
v0.57
Merge branch 'next'
Merge branch 'master' of https://github.com/ceph/ceph