Always explicitly specify --output for ITs

With a revamp of the examples in [#1275](https://github.com/apache/incubator-beam/pull/1275) the `--output` parameter becomes mandatory rather than defaulting to the temp location (this was weird anyhow).

The ITs all fail because they leave this value implicit, as in https://builds.apache.org/job/beam_PreCommit_MavenVerify/4635/org.apache.beam$beam-examples-java/testReport/junit/org.apache.beam.examples/WindowedWordCountIT/testWindowedWordCountInBatch/

I believe this requires Jenkins access. I could be wrong.

Imported from Jira [BEAM-910](https://issues.apache.org/jira/browse/BEAM-910). Original Jira may contain additional context.
Reported by: kenn.

Start with the WindowedWordCountIT testWindowedWordCountInBatch failure in the linked beam_PreCommit_MavenVerify report, then locate the integration-test invocations that leave --output implicit. Check the Maven verification result after making each IT specify an output location; done means the affected ITs no longer fail for a missing --output value.

Written by the indexing model from the issue text.