systemd: do not pass --cluster option

systemd: do not pass --cluster option

we do not encourage user to run multiple cluster using different
cluster name since 8fe460e7562b28e007179eeb533ef68a3e99b4c8, and
the recommended way is to use cephadm. see also
https://docs.ceph.com/en/latest/rados/configuration/common/#naming-clusters-deprecated,
so let's stop passing `--cluster` in systemd service units. there
are more places where we are still feeding this optoin to command line
tools or services. we should do the cleanup piecemeal to reduce the
potential risk.

FWIW, if not specified, the cluster name is "ceph" by default.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>