Allow enabling secure mode with Kerberos on HDFS

[lfrancke]

As a user/admin I'd like to be able to enable secure mode in HDFS using Kerberos so I can authenticate all requests against HDFS.

This is done when

• a CRD change(s) has been proposed and reviewed on how to enable Kerberos
• it uses the functionality provided by the secret operator to provision Kerberos keytabs Kerberos keytab provisioning secret-operator#4
• documentation is added
• integration tests are added
• examples are added
• feature tracker is updated