dist: do not always restart jmx on shutdown
dist: do not always restart jmx on shutdown

Restart=always leads to the following loop:

 1. scylla terminates abnormally
 2. scylla-jmx sees that, and terminates
 3. systemd sees that scylla-jmx terminated, and restarts it.
 4. scylla-jmx requires scylla, so systemd starts it.
 5. goto 1.

To prevent the loop, set Restart=on-abnormal; systemd will restart scylla-jmx
if some JVM bug got it killed, but not otherwise.

The downside to this patch is that if scylla-server goes down, so does
scylla-jmx, but if scylla-server is then restarted, scylla-jmx stays down.
To get scylla and scylla-jmx to start together, we need to create
scylla.service that requires both of them.

scylladb/scylla-jmx,scylladb/scylla-jmx,scylladb/scylla-jmx