VPRN template: add support for `label-mode`

label_mode defined via inventory:
- next-hop
- vrf

geant/gap_ansible/roles/bc_templates/routers/nokia/router/vprn.j2

@@ -11,6 +11,9 @@
         {% if vprn.ecmp is defined %}
         <ecmp>{{ vprn.ecmp }}</ecmp>
         {% endif %}
+        {% if vprn.label_mode is defined %}
+        <label-mode>{{ vprn.label_mode }}</label-mode>
+        {% endif %}
         <bgp-ipvpn>
             <mpls>
                 <admin-state>enable</admin-state>