Verify Help button redirects to the right URL

When choosing "Help", in the top bar user drop-down menu,
the browser should redirect to http://docs.openstack.org/
Different distributions might customize the redirection URL,
but the above should be the default.

I added test_dashboard_help_redirection.py, go_to_help_page
method in basepage and the help_url value to horizon.conf
(also forced a change in config.py)

Partially implements blueprint: selenium-integration-testing
Closes-Bug #1350408

Change-Id: I64e661690ee9c26eb9d69092024465ab1c15b8a1
Merge "Verify Help button redirects to the right URL"