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 would name this `SELENIUM_BROWSER_VERSION` otherwise it could be misleading. Similarly, the one above could become `SELENIUM_BROWSER_NAME` for consistency.