Adding Appium support for Saucelabs

Capabilities that we need to run Appium on Saucelabs https://support.saucelabs.com/entries/51370580-Android-Desired-Capabilities

This is for issue https://github.com/mozilla/pytest-mozwebqa/issues/122
@davehunt here are the capabilities needed for Saucelabs to run with Appium https://support.saucelabs.com/entries/51370580-Android-Desired-Capabilities
By working with this, I don't know if there's something else that should be specified when we want to run with Appium. When I  test locally with Appium, I can play around with the Capabilities, but Saucelabs requires all of them to be specified so it can run with Appium