[systemd] Use a private /tmp directory

This creates an isolated tmp directory for the kibana service.  Reads
and writes to /tmp will end up in /tmp/systemd-private-*-kibana.service-*/tmp,
isolated to the current process.

src/dev/build/tasks/os_packages/service_templates/systemd/usr/lib/systemd/system/kibana.service

@@ -8,6 +8,7 @@ After=network-online.target
 Type=simple
 User=kibana
 Group=kibana
+PrivateTmp=true
 
 Environment=KBN_HOME=/usr/share/kibana
 Environment=KBN_PATH_CONF=/etc/kibana