Add option to oo-register-dns to use GSS-TSIG Kerberos credentials to bind.

Hello,

please consider this patch which adds option -g to oo-register-dns and passes it to nsupdate to use the GSS-TSIG to bind to named.

Also, I have noticed that oo-register-dns does not observe content of /etc/openshift/plugins.d/openshift-origin-dns-nsupdate.conf when setting the defaults. So for example, if you have BIND_SERVER set there to non-localhost machine, you still need to pass the -s option to oo-register-dns.

If you'd prefer oo-register-dns getting the default behaviour from openshift-origin-dns-nsupdate.conf, I could make it parse this file and use that -g option to nsupdate automatically whenever BIND_KRB_PRINCIPAL and BIND_KRB_KEYTAB is set.

Thank you,

Jan