Added crashdump action.

[petevg]

Zips up the matrix log, and calls out to the juju-crashdump plugin to
fetch logs and other relevant files from the model.

@johnsca @bcsaller

[petevg]

@johnsca Ugh. I just realized that there's an issue with this: if a test raises an exception, which is a case where we very much want a crashdump, then we'll never actually run the crashdump task.

Our options are:

a) refactor so that we can run execute_process outside of a task (it's just a wrapper around the asyncio tools, so writing another wrapper wouldn't be too much of a big deal).
b) Come up with a "finally" sort of action, which always executes, even if we've been interrupted by a TestFailure.

[petevg]

Closing. Will resubmit with a crashdump that actually works like we want it to ...