Listen on all available interfaces by default

As described in https://github.com/xaque208/puppet-unbound/issues/12, in order to properly support hosts with more than one IP address, Unbound should be configured to listen on each available interface.

This pull request uses a custom Facter fact to gather the available ip addresses into a string, then the Puppet manifest splits that string into an array for use in the template. On my test box, `unbound.conf` looks a bit like this.

    server:
      interface: 127.0.0.1
      interface: 10.0.0.13
      interface: 10.0.0.92
      interface: ::0

Of course, access controls are left alone because access should still be disabled by default, and require the admin to specify certain access controls.

*Ack, I just realized that I sourced this branch from another change I requested. It's late and I don't want to rebranch right now, but let me know if you'd rather have it separated out of this pull request.*