bc_templates/bgp_group.j2: add group description

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

@@ -2,6 +2,9 @@
                 <group alu:operation="replace">
                     <group-name>{{ group.name }}</group-name>
                     <admin-state>{{ group.admin_state | default('enable') }}</admin-state>
+                    {% if group.description is defined %}
+                    <description>{{ group.description }}</description>
+                    {% endif %}
                     {% if group.auth_key is defined %}
                     <authentication-key>{{ group.auth_key }}</authentication-key>
                     {% endif %}