As advised with Tobias, having a "read legacy" annex
function to readSettings() is the simplest solution for
backward compatibility with old VCS settings.
readLegacySettings() is useful when refactoring a Vcs
settings class to make it inherit VcsBaseClientSettings.

Change-Id: I96e3f67921f607bf010688d023098114e0e91035
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>