Add explicit wait for height of the promo box on DiscoveryPane page

test_that_mission_statement_is_on_addons_home_page have failed on addons.dev: http://selenium.qa.mtv2.mozilla.com:8080/view/AMO/job/amo.dev/1363/HTML_Report/
From the screenshot it looks like the DiscoveryPane page was not completely loaded, as the promo box is not displayed. 
I added a wait for the height of the promo box, similar to: https://github.com/mozilla/Addon-Tests/blob/master/pages/desktop/home.py#L52, to make sure the page is ready for testing.