Add the ability to control how much info is coming out from failures using Py.test --tb feature

Example usage is

`./go //py:marionette_test:run --traceback=no`

This will not show failure tracebacks. The default remains as long.
Valid traceback values are long, short, line, native and no

Author: AutomatedTester

rake-tasks/crazy_fun/mappings/python.rb

@@ -159,7 +159,8 @@ def handle(fun, dir, args)
           mark_filter = "-m=\"not ignore_#{browser_data[:ignore]}\"" if browser_data[:ignore]
           pytest_args += [mark_filter]
           keyword_filter = "-k=" + ENV['method'] if ENV['method']
-          pytest_args += [keyword_filter]
+          traceback_level = "--tb=" + ENV['traceback'] if ENV['traceback']
+          pytest_args += [keyword_filter, traceback_level]
           pytest_args += ["--junitxml=build/test_logs/python-#{browser}-#{Time.now.to_i}.xml"]
           mkdir_p "build/test_logs"
           sh pytest_args.join(' '), :verbose => true