cmd/storagenode: fix failing supervisord "processes" eventlistener

/bin/stop-supervisor fails in posix shell since the standard read utility
takes at least one variable's name as argument.

Changing the header #!bin/sh to #!/bin/bash fixes this issue.
`read` with no variable's name works in bash.

Looks like the shell in alpine isn't POSIX-compliant so we didn't
encounter this issue on alpine.

Also, I changed the name from "processes" to "processes-exit-eventlistener"
to make it clearer in the logs since supervisord spawns event listeners as
separate processes.

Change-Id: Ife9378c2013e2eb54f2adcd52a163d64eaacbbab

cmd/storagenode/docker-base/bin/stop-supervisor

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 printf "READY\n";
 

cmd/storagenode/docker-base/etc/supervisor/supervisord.conf

@@ -35,6 +35,6 @@ stdout_logfile_maxbytes=0
 stderr_logfile=/dev/stdout
 stderr_logfile_maxbytes=0
 
-[eventlistener:processes]
+[eventlistener:processes-exit-eventlistener]
 command=/bin/stop-supervisor
 events=PROCESS_STATE_EXITED, PROCESS_STATE_FATAL