Combine changes for BZ955602 and BZ655643 (dns-nsupdate config and exceptions)

* Raise proper exeception: DNSException
* Check access and presence of KRB5 keytab file
* Do not initialize TSIG variables if GSS-TSIG is provided

Replace PR2199, 2198 and 2260
Merge pull request #2269 from markllama/bugs/BZ955602

Merged by openshift-bot