add log-prefix to syslog

As per NE request: optional 'log-prefix' config to the syslog section.

geant/gap_ansible/roles/base_config/templates/routers/nokia/syslog/syslog.j2

@@ -14,6 +14,9 @@
     {% if server.severity is defined %}
     <severity>{{ server.severity }}</severity>
     {% endif %}
+    {% if server.log_prefix is defined %}
+    <log-prefix>{{ server.log_prefix }}</log-prefix> 
+    {% endif %}
   </syslog>
 {% endfor %}
 {# LOG FILES #}