Remove indefinite caching of key provider uri in DFSClient

  /**
   * The key provider uri is searched in the following order.
   * 1. If there is a mapping in Credential's secrets map for namenode uri.
   * 2. From namenode getServerDefaults rpc.
   * 3. Finally fallback to local conf.
   * @return keyProviderUri if found from either of above 3 cases,
   * null otherwise
   * @throws IOException
   */
  URI getKeyProviderUri() throws IOException {
    if (keyProviderUri != null) {
      return keyProviderUri;
    }
    // Lookup the secret in credentials object for namenodeuri.
    Credentials credentials = ugi.getCredentials();
   ...
   ...