Add support for GSS-TSIG configuration.

Add support for configuration variables CONF_BIND_KRB_KEYTAB
and CONF_BIND_KRB_PRINCIPAL to configure Kerberos based nsupdate
operation in openshift-origin-dns-nsupdate.conf.

Since the openshift-origin-dns-nsupdate.conf supports the BIND_KRB_KEYTAB
and BIND_KRB_PRINCIPAL and works with them, this patch just closes the loop
by making it possible to install/setup the GSS-TSIG nsupdate via openshift.sh.

I have been using this for a while and it seems to achieve the goal fine.

The only drawback I've hit so far is oo-accept-broker which does not understand
the BIND_KRB_* values. I plan to submit patch for oo-accept-broker soon.

Thank you,

Jan