Fix templating error in radvd.conf

When configuring radvd service an undefined object error is thrown.
The data function in RadvdService returns a dict with ifaces. The radvd
template however, expects a dict with a values key.

I changed the key in the template because "ifaces" is way more descriptive
than "values".

Original Error that is fixed:
Traceback (most recent call last):
  File "/opt/core/venv/lib/python3.9/site-packages/core/configservice/base.py", line 471, in render_template
    return self._render(template, data)
  File "/opt/core/venv/lib/python3.9/site-packages/core/configservice/base.py", line 439, in _render
    return template.render_unicode(
  File "/opt/core/venv/lib/python3.9/site-packages/mako/template.py", line 444, in render_unicode
    return runtime._render(
  File "/opt/core/venv/lib/python3.9/site-packages/mako/runtime.py", line 874, in _render
    _render_context(
  File "/opt/core/venv/lib/python3.9/site-packages/mako/runtime.py", line 916, in _render_context
    _exec_template(inherit, lclcontext, args=args, kwargs=kwargs)
  File "/opt/core/venv/lib/python3.9/site-packages/mako/runtime.py", line 943, in _exec_template
    callable_(context, *args, **kwargs)
  File "/opt/core/venv/lib/python3.9/site-packages/core/configservices/utilservices/templates/etc/radvd/radvd.conf", line 2, in render_body
    % for ifname, prefixes in values:
TypeError: 'Undefined' object is not iterable

daemon/core/configservices/utilservices/templates/etc/radvd/radvd.conf

@@ -1,5 +1,5 @@
 # auto-generated by RADVD service (utility.py)
-% for ifname, prefixes in values:
+% for ifname, prefixes in ifaces:
 interface ${ifname}
 {
         AdvSendAdvert on;