stbt-docker: Add support for Microsoft Windows
stbt-docker: Add support for Microsoft Windows

Assuming that you've installed docker with docker-toolbox and are running
in a fairly standard configuration.

Notes:

* git for Windows checks out symlinks in git repos as normal files.  We've
  had to do the symlink chasing to find the stbt.conf here ourselves.
* `os.execvp` is present on Windows, but creates a new process killing the
  old one, rather than just replacing the existing process.  This caused
  the terminal that calls stbt-docker to think that the command is done.
* Windows has `\` as a path separator rather than `/` but we still have
  to be careful to pass `/` to the docker command line.

stb-tester/stbt-docker,stb-tester/stbt-docker