qa/cephadm: fix hardcoded 'sshd' unit name in setup_ca_signed_keys

qa/cephadm: fix hardcoded 'sshd' unit name in setup_ca_signed_keys

setup_ca_signed_keys() hardcoded `systemctl restart sshd`, which fails
on Debian/Ubuntu where the SSH server unit is named `ssh.service`
rather than `sshd.service`. Try `ssh` first, fall back to `sshd` if
that fails, grouped so the fallback only fires on restart failure and
doesn't mask a failed config write.

Fixes: https://tracker.ceph.com/issues/77991
Signed-off-by: Redouane Kachach <rkachach@ibm.com>