Configurable Selectors in Integration Tests

Horizon's integration tests are written assuming the default theme is being used. Other themes might have a different structure of the underlying HTML, and would require modifying the selectors used in the tests to make them work properly. This proposes to make those selectors configurable in the local-horizon.conf file, so that the integration tests can be run with a non-default theme as well.

For example, in regions/bars.py the TopBarRegion class has selectors that assume the user drop-down menu is on the right hand side: _user_dropdown_menu_locator = (by.By.CSS_SELECTOR, '.nav.navbar-nav.navbar-right'), however in the Red Hat theme that menu is on the left, and that selector needs to be something like '.header .navbar .navbar-inner'. We could do this by introducing a [selectors]top_bar_region__user_dropdown_menu_locator setting, that would default to the default theme, but could be changed for testing with any other theme.