Automatically install and configure DNS root hints

[nicwaller]

Here is a possible solution to issue #7 about not having root hints available. I tried to copy the existing patterns as much as possible.

This patch relies on the use of exec to download the latest root hints from IANA. I think it's reasonable to assume that a DNS server has curl available and can contact the IANA webserver. The exec runs only if the hints file is missing, which is normally only on the first run.

Originally I was going to add extra parameters for customization, but I ended up keeping it simple after running into a couple problems:

• Unbound is chrooted, so it can't access files outside of the /etc directory. There isn't really any reason for somebody to customize the path to the root hints file.
• I was going to add a parameter that makes fetching root hints optional, but I wasn't sure how to handle the requires dependency correctly with the cron resource wrapped inside an if {} statement. Maybe the before metaparameter would work better?
• I was considering adding a cron resource that would update the root hints file, but I don't know of an elegant way to avoid the "thundering herd" problem that would result in a lot of simultaneous queries to the root hints servers if many servers were using this module.

[nicwaller]

This patch is was developed with Puppet 3.2.4 on Scientific Linux 6.4 (osfamily=redhat) and selinux in enforcing mode.