add a flag for marketplace tests

Once you set the correct time on the pandaboard, the Marketplace loads, but the  test_search_marketplace_and_install_app.py is failing because 1) self.wait_for_element_displayed(*self._search_button) doesn't return in time, and 2) if you comment that out, you'll notice that the screen refreshes.

In light of trying to get tests green on tbpl, I'm adding a flag to disable these tests.

Link to marketplace bug: https://bugzilla.mozilla.org/show_bug.cgi?id=832478