Support Kerberos connection

[evren]

Controller Service should provide additional options for Kerberos authentication.

[PaulJackson123]

This could mean a number of things.

1. A Nifi equivalent of the --krg5 and --krb5-disable-rdns options
2. Using the Kerberos credentials of the user that logged in to Nifi to access Stardog
3. Integration with the Keytab Controller Service

Option 1 would always use the credentials of the user that launched the Nifi service.
Option 2 doesn't seem like something Nifi has written about in the doc or blogs. I could be missing something.
Option 3 (documented here) lets different groups access Stardog as different users by associating each group with their own keytab file.

What's the utility of option 1? It saves the user from entering the credentials one time, but that can be done without Kerberos by using a controller service.