Implement a utility HTTP server/interface

[randomir]

Local web server is needed to handle redirects from Leap and exchange the authorization code for token (see #561), without further user intervention (like manually pasting the code).

We already implement a simple web server in https://github.com/dwavesystems/dwave-inspector, one that is based on the built-in WSGIServer and WSGIRequestHandler, and that it runs Flask.

We might want to either:

• move the dwave-inspector server implementation over to the cloud-client (since it's a dependency anyway); or
• implement a new server based on off-the-shelf werkzeug.run_simple, or similar.