--worker_harness_container_image should issue a warning if not used with --runner DataflowRunner

[kennknowles]

Running
python -m apache_beam.examples.wordcount --output counts --worker_harness_container_image <container_name>
doesn't issue a warning saying that the job is being run by DirectRunner (default) and therefore no worker_harness_container_image will be used.

Imported from Jira BEAM-2048. Original Jira may contain additional context.
Reported by: mariagh.