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
So appium_version is required to indicate that we're targetting Appium? I think it would be simpler to default to the latest version of Appium if possible, and use some other way to indicate that the user wants the tests to run against Appium.