Support for extra appium capabilities at the scenario level

[nodlesh]

We may need the ability to have extra appium capabilities set at the scenario level. There are cases arising where having the biometrics turned off or on allowTouchIdEnroll=true and the device language set setCapability("language", "en") before driver is instantiated for the scenario.

Maybe a @extra_config_<test_identifier> tag on the scenario in the feature file, and if it is there, we load extra capabilities from a file called <test_identifier>.json located in the features folder. It would do this in the before_scenario() function in the environment.py file before driver is created.