cephadm: run tcmu-runner through script to do restart on failure

cephadm: run tcmu-runner through script to do restart on failure

Currently, cephadm runs tcmu-runner as a background
process inside the unit file deployed for iscsi
(rbd-target-api is the primary process). This means
if tcmu-runner crashes for whatever reason, systemd
will not attempt to restart it. This commits sets
up a script to serve as the container entrypoint
for the tcmu-runner container that will run
tcmu-runner and also restart it on failure
(unless there are too many failures in a short
period, at which point it gives up).

The hope is to eventually drop use of this script
for a better solution in squid onward, but this
should be helpful on older releases (quincy and
pacific at least) where we won't be able to
bring that better solution

Fixes: https://tracker.ceph.com/issues/61667
Signed-off-by: Adam King <adking@redhat.com>
(cherry picked from commit 47eb6b3f62afe993073429b02051ae0343d7aea3)

Conflicts:
	src/cephadm/tests/test_cephadm.py