Set LOGOUT_URL properly in horizon settings

LOGOUT_URL depends on the root_path just like LOGIN_URL was is already set in
the template.

Without this change, a user whose session timeouts was redirected to
/auth/logout (404 error) instead of /horizon/auth/logout.

Change-Id: I4f7c7ab4d20eed514c48d79d78980d7415d70be5
Merge "Set LOGOUT_URL properly in horizon settings"