exit: only send RAs on interfaces with FHRP addresses

These are the ones we are router for.

roles/exit/templates/radvd.conf.j2

@@ -1,4 +1,5 @@
 {% set my_vlans = interfaces | selectattr('parent') | selectattr('parent.name', '==', 'bridge')
+    | selectattr('count_fhrp_groups', '>', 0)
     | selectattr('untagged_vlan') | map(attribute='untagged_vlan') -%}
 
 # Send IPv6 RAs from virtual router IP for each network. Also set DNS options.