Fix typos in config file template for infra-host-ttl

Commit 628fc23f364321d7e3e58191a638304bcb658f62 introduced errors due to typos in unbound.conf, mostly mixed up underscores and hyphens. Here's a pull request to correct those issues.

Also, better to use @variable in erb templates so that variables set to undef are properly set as undefined instead of being rendered as the string "undef."

This is the error I got:

    Error: Could not retrieve catalog from remote server: Error 400 on SERVER: Failed to parse template unbound/unbound.conf.erb:
      Filepath: /usr/lib/ruby/site_ruby/1.8/puppet/parser/templatewrapper.rb
      Line: 81
      Detail: Could not find value for 'infra_host' at /etc/puppet/modules/unbound/templates/unbound.conf.erb:38
     at /etc/puppet/modules/unbound/manifests/init.pp:54 on node scientific4.localdomain
    Warning: Not using cache on failed catalog
    Error: Could not retrieve catalog; skipping run