fixes for register_dns

- do not attempt to update dns records for hosts that are not a member
  of the host domain ($dns_infrastructure_zone if set, otherwise
  $domain)
- fix bug where nsupdate was being run against the $domain variable
  if $dns_infrastructure_zone is set.
- remove extra spaces output from register_dns.erb template
Merge pull request #339 from detiber/register_dns_fixes

fixes for register_dns