title: AssertionError: INTERNAL: No default project is specified
body: <p>New to airflow. Trying to run the sql and store the result in a BigQuery table. </p>
  
  <p>Getting following error. Not sure where to setup the default_rpoject_id.</p>
  
  <p>Please help me. </p>
  
  <p>Error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "/usr/local/bin/airflow", line 28, in &lt;module&gt;
      args.func(args)
    File "/usr/local/lib/python2.7/dist-packages/airflow/bin/cli.py", line 585, in test
      ti.run(ignore_task_deps=True, ignore_ti_state=True, test_mode=True)
    File "/usr/local/lib/python2.7/dist-packages/airflow/utils/db.py", line 53, in wrapper
      result = func(*args, **kwargs)
    File "/usr/local/lib/python2.7/dist-packages/airflow/models.py", line 1374, in run
      result = task_copy.execute(context=context)
    File "/usr/local/lib/python2.7/dist-packages/airflow/contrib/operators/bigquery_operator.py", line 82, in execute
      self.allow_large_results, self.udf_config, self.use_legacy_sql)
    File "/usr/local/lib/python2.7/dist-packages/airflow/contrib/hooks/bigquery_hook.py", line 228, in run_query
      default_project_id=self.project_id)
    File "/usr/local/lib/python2.7/dist-packages/airflow/contrib/hooks/bigquery_hook.py", line 917, in _split_tablename
      assert default_project_id is not None, "INTERNAL: No default project is specified"
  AssertionError: INTERNAL: No default project is specified
  </code></pre>
  
  <p>Code: </p>
  
  <pre><code>sql_bigquery = BigQueryOperator(
          task_id='sql_bigquery',
          use_legacy_sql=False,
          write_disposition='WRITE_TRUNCATE',
          allow_large_results=True,
          bql='''
              #standardSQL
                  SELECT ID, Name, Group, Mark, RATIO_TO_REPORT(Mark) OVER(PARTITION BY Group) AS percent FROM `tensile-site-168620.temp.marks`
                  ''',
          destination_dataset_table='temp.percentage',
          dag=dag
          )
  </code></pre>