site provider: link for default site is 000-default

nxensite and nxdissite rename any site named 'default' to
'000-default' when creating the symlink. This causes a reload
of nginx on every chef-client run when you have a site named
'default'. This patch makes the openresty_site provider aware
of such behavior in nx{en,dis}site.
Merge pull request #5 from sdelano/sd/nxensite-default-priority

site provider: link for default site is 000-default