Attempt to load webdriver parameters from os.environ

While testing with saucelabs, I found it would be helpful for pytest-mozwebqa to use `SELENIUM_*` environment variables set by saucelabs.  The following change sets the default value for `--browsername`, `--browserver`, and `--platform` using `os.environ`.
Hmm.. I don't think they thought through the `SELENIUM_VERSION` one... If you think compatibility with the Saucelabs plugin is most important, then I can overlook this poor variable naming.