Enable nginx server listening on IPv6 networks

By default nginx will now listen for requests both on IPv4 and on IPv6
networks. You can still specify your own 'listen' directives, of course.
Support IPv6 OCSP resolver

This is a simple check to determine if an IP address is IPv4 or IPv6 and
surround the latter with brackets, as per nginx configuration
requirements.

This is needed in case that DNS server selected as OCSP resolver is an
IPv6 DNS server.
Merge pull request #4 from drybjed/ipv6

Improved IPv6 support in nginx role