Update UI to use new stack artifact kerberos descriptor api endpoints

The Ambari UI should now use the new api endpoints for obtaining the kerberos descriptor for stacks and stack services.

Currently the UI uses temporary fields in the stack version and stack service endpoints.

Old api call to get stack descriptor:

api/v1/stacks/HDP/versions/2.2?fields=Versions/kerberos_descriptor

Old api call to get service descriptor:

api/v1/stacks/HDP/versions/2.2/services/HDFS?fields=StackServices/kerberos_descriptor

Usages of the above api should be replaced with the new stack artifact endpoints:

New way to get stack descriptor:

api/v1/stacks/HDP/versions/2.2/artifacts/kerberos_descriptor

New way to get service descriptor:

api/v1/stacks/HDP/versions/2.2/services/HDFS/artifacts/kerberos_descriptor

These new calls will return the requested kerberos_descriptor resource which has the format:

{
  "href" : ".../api/v1/stacks/HDP/versions/2.2/services/HDFS/artifacts/kerberos_descriptor",
  "Artifacts" : {
    "artifact_name" : "kerberos_descriptor",
    "service_name" : "HDFS",
    "stack_name" : "HDP",
    "stack_version" : "2.2"
  },
  "artifact_data" : {
    ...
  }
}

The descriptor information is contained in the "artifact_data" field.

This task is dependent on the work done for: https://issues.apache.org/jira/browse/AMBARI-9028