qa: ceph tell must retry on ENXIO

It is expected for ceph tell to fail with ENXIO if the daemon it is
trying to join is not ready for some reason. This should be handled as a
transient error instead of a fatal error.

Add two shell functions to help with retry. They may prove useful if
other cases requiring a few retries show up.

http://tracker.ceph.com/issues/9655 Fixes: #9655

Signed-off-by: Loic Dachary <loic-201408@dachary.org>