Update the Mobile tests to work with Firefox Accounts and Appium

This is for https://github.com/mozilla/marketplace-tests/issues/500

Pytest-mozwebqa needs to be updated first : https://github.com/mozilla/pytest-mozwebqa/pull/127

Here is the Jenkins build http://selenium.qa.mtv2.mozilla.com:8080/job/marketplace.dev.mobile.saucelabs.appium/