No tests for Kerberos

[wouterdebie]

Moved this into and issue (@bolkedebruin).

The issue is that there are no tests for Kerberos. I'm not sure if the minicluster can do this (@ravwojdyla) and how to setup a Kerberos realm in Travis.

[bolkedebruin]

There are a two main conditions that need to be met to allow for kerberos integration testing:

1. A functional KDC
2. Hadoop running secured (in case of snakebite just HDFS is required)

To get a functional KDC running there seem to be several options:

1. Full KDC in Docker (something like this: https://github.com/tillt/docker-kdc).
2. MiniKdc from Apache Hadoop (discussion here: https://issues.apache.org/jira/browse/HADOOP-9848)
3. MiniKdc from the Spring Framework (http://docs.spring.io/spring-security-kerberos/docs/current/api/org/springframework/security/kerberos/test/MiniKdc.html derived from the Hadoop version)

In any case you would need to setup principals and key tabs. For option 1 docker-in-docker for travis is required). For the other options further research needs to be done.