Ensure there's always an http server object

It's possible for a test to complete before python gets around to
actually starting the webapp run thread.  Make sure that the
object referenced in the stop method is always instantiated to
avoid "AttributeError: 'WebApp' object has no attribute 'server'"
errors.

Also, mark the webapp thread as daemon on general principle.

Change-Id: I9b4c0c20a98d5429c299748178e31baf8b6912d9