Allow the testenv client to dictate the worker timeout

The testenv client is privileged to information about jobs that the
worker is not aware of, so allow the client to send us the appropriate
timeout.

This also changes the format of the the data sent to us from the client
to a json representation of a python dict.

Change-Id: If3f082c04d937f0d5baa09cf002aef797eebcbe5
Merge "Allow the testenv client to dictate the worker timeout"