Use hiera_array when looking up nameservers

Using hiera_array rather than just hiera will perform an array merge
rather than just getting the most specific hieradata for this variable.

We want to be able to define nameservers across multiple hiera files.
Add GDS nameserver to Jumpbox

This will allow us to resolve GitHub Enterprise.
Merge pull request #656 from alphagov/add-github-gds-nameserver

Add GDS nameserver