Bug 1162901 - ensure correct dns key algo used

https://bugzilla.redhat.com/show_bug.cgi?id=1162901

- Previously register_dns.pp used pick to choose between
  bind_key_algorithm and dns_infrastructure_key_algorithm, but since
  they both had default values of HMAC-MD5,
  dns_infrastructure_key_algorithm was always used.
- show list of skipped hostnames in nsupdate command (debug output)
- fail if bind_key is not set and dns_infrastructure_zone is not set
- fail if dns_infrastructure_key is not set and dns_infrastructure zone
  is set
Bug 1162454 - default hostnames should use infra zone if provided

https://bugzilla.redhat.com/show_bug.cgi?id=1162454

- If no hostname is provided and dns_infrastructure_zone is configured
  the default hostnames should use dns_infrastructure_zone instead of
  domain variable