Add ipv6 support to erchef config template
If ipv6 mode is set, ensure nginx is in ipv6 mode

However, don't override nginx setting if it is set to ipv6 but the
the rest of the server is in ipv4 mode
Add sysctl recipe and force net.ipv6.bindv6only to 0

- Only require dual ipv6/4 bindings if this is an ipv6 configuration
- Force always successful for syctl -p to work around bad centos entries