Make KerberosKeyTabUserService respect NIFI_ALLOW_EXPLICIT_KEYTAB env variable

There was a validation for the plain kerberos properties in kafka processors that prevented using keytabs when NIFI_ALLOW_EXPLICIT_KEYTAB was false. It was removed in NIFI-11202, but I think it would still be useful for KerberosKeyTabUserService, which incorporates the same properties.

I think it can be cherry picked to the support branch.