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 reading that page, deviceName is not specific to Appium and shouldn't be in an 'appium' group. If specifying an Appium version is how to indicate you want your tests run using Appium in Sauce Labs then we should at least document that in the README and suggest the minimum version number. I don't think this current solution is particularly user friendly though. I think having an --appium flag on the command line might be a little clearer, though I confess that I haven't given it a lot of thought.