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`.
Hah!  I agree, the naming isn't very clear.  I'll update the PR with other suggestions.  Thanks for the feedback.