Implement Nomad Task Driver support (0.9.x)

[justenwalker]

Damon was developed long before Nomad had the concept of a Task Driver. It was always the plan that some day we'd like to contribute this back to Nomad as a part of the engine. Now since Nomad 0.9.0 preview supports pluggable task drivers, we can use that as another way to run windows executables with Damon.

This alternate mode needs documentation as well as implementation.

Some Open Questions still:

• How will Damon report metrics as a Task Driver? Right now it is done as another separate task service and advertised port.
• Will the task driver have access to the full job spec? Possibly to read in more details about the job that are not present entirely in the environment variables.
• How does the task driver log? Is the log aggregated in the nomad client logs? Originally we liked the log to be along-side the task since it was convenient for log shipping application logs with damon events to correlate problems. I'm sure we can continue to log to file ourselves with rotate logs; but maybe make it configurable.

[justenwalker]

Progress tracked here: #17