qa/workunits/rbd: reduce randomized sleeps in live import tests

qa/workunits/rbd: reduce randomized sleeps in live import tests

These tests were tuned for slower hardware than what we have now.
Currently "rbd migration execute" always finishes (successfully) before
the NBD server is killed.

Fixes: https://tracker.ceph.com/issues/74669
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 592e3a9846b130c7321481f8b2bf9dba2fb05195)

Conflicts:
	qa/workunits/rbd/cli_migration.sh [ commit afc89fdde80f
	  ("qa/workunits/rbd: add test_import_nbd_stream_disconnected()")
	  was originally skipped due to NBD stream not being in squid at
	  the time ]