Fix filename and next-server options in subnet.

dhcpd/files/subnet.jinja

@@ -15,10 +15,10 @@
 {{ intendation }}  option broadcast-address {{ config['broadcast_address'] }};
 {%- endif %}
 {%- if config.has_key('filename') %}
-{{ intendation }}  option filename {{ config['filename'] }};
+{{ intendation }}  filename "{{ config['filename'] }}";
 {%- endif %}
 {%- if config.has_key('next_server') %}
-{{ intendation }}  option next-server {{ config['next_server'] }};
+{{ intendation }}  next-server {{ config['next_server'] }};
 {%- endif %}
 {%- if config.has_key('routers') and config.routers is string %}
 {{ intendation }}  option routers {{ config.routers }};