Trigger example in the cookbook failed to run on Dataflow

The example org.apache.beam.examples.cookbook.TriggerExample failed to run on Dataflow. Big query returns failure code 409 indicating it's trying to write to an non-empty table but WriteDisposition is set to WRITE_EMPTY (the default value). The example fails with this error even if the output table does not exist before the example is run. 

In fact, the four pipelines corresponding to the four trigger types write to the same table. The pipeline that finishes first will create the table and insert data into it. But the remaining ones can only _append_ to that table. Hence, the problem can be avoided by setting WriteDisposition to WRITE_APPEND.

Imported from Jira [BEAM-3844](https://issues.apache.org/jira/browse/BEAM-3844). Original Jira may contain additional context.
Reported by: Zhaozhi.