Allow enabling and disabling 'default' nginx_site

The nxensite and nxdissite scripts prepend a '000' priority to the
symlink name for the default site. Adding this to the guard conditions
for the nginx_site definition lets us enable and disable the default
site properly.
Merge pull request #90 from nwjsmith/master

[COOK-2398] Allow nginx_site to manage the default site