Fix cron subprocess terminates silently

We noticed the subprocess that runs our 'cron' sometimes quits silently;
one possibility we see is that OOM kills it. In any case, we now also
check for the existence of the commands `sleep` or `timeout` in the
process table (`ps`) via the `livenessProbe`.

Address #60

kubernetes/dspace-deployment.yaml

@@ -74,6 +74,11 @@ spec:
             path: /repository/xmlui/
             port: 443
             scheme: HTTPS
+          exec:
+            command:
+              - /bin/sh
+              - -c
+              - ps aux | grep -v grep | grep -E "sleep|timeout"
           initialDelaySeconds: 600
           periodSeconds: 15
           timeoutSeconds: 3