Add html reports to report action in coverage extension.

This makes a few minor cleanups and 2 additions to the
coverage extension. First, it adds a new optional field to the
report action: 'html'. This will generate an html report instead
of either the text or xml reports that the extension can
generate.

The second change is that it adds a return for the stop action.
Now upon successful completion of the stop action the directory
where the coverage data files will be returned.

Change-Id: If1aa25fc7237e9bb5100d2a4a8e560f0a68eba61
Merge "Add html reports to report action in coverage extension."