Account for case when BIND_KRB_* is used in openshift-origin-dns-nsupdate.conf.

When the GSS-TSIG is configured for nsupdate, BIND_KRB_KEYTAB and
BIND_KRB_PRINCIPAL are used in /etc/openshift/plugins.d/openshift-origin-dns-nsupdate.conf instead of BIND_KEYNAME and BIND_KEYVALUE.

We need to allow the check to pass when one pair of values is used and not the other one, and we probably want to use nsupdate -g in case GSS-TSIG is configured (we assume the kinit was already done).

This patch amends https://github.com/openshift/openshift-extras/pull/26 which was already merged, so that configuring the GSS-TSIG with openshift.sh does not lead to broker oo-accept-broker.

Thank you,

Jan