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`.
I can make those changes.  However, I chose the environment variables because those are the values used by the [saucelabs jenkins plugin](https://docs.saucelabs.com/ci-integrations/jenkins/).  How would you like me to proceed?