NIFI-6808 Adding KeytabCredentialService to HortonworksSchemaRegistry…

[bbende]

… and setting dynamic JAAS config property on client

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Enables X functionality; fixes bug NIFI-YYYY.

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[jtstorck]

Reviewing...

[jtstorck]

Thanks for the contribution, @bbende!

Tested this locally with tillt/docker-kdc, Hortonworks Schema Registry 0.8.1, and NiFI 1.11.0-SNAPSHOT.

Verified that a KeytabCredentialsService configured with a principal and keytab can be used with the HortonworksSchemaRegistry controller service. This was tested with a valid principal and worked, and also tested with an invalid principal, which failed as expected.

During the failure while using the invalid principal, I verified in the logs that the referenced schema could not be retrieved, with a root cause of a 401 from the Hortonworks Schema Registry service.

+1, LGTM! Will merge.