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.
Merge pull request #26 from adelton/enterprise-1.2

Add support for GSS-TSIG configuration.