[config] Put "Instance" and "Instance data" directory within cubiweb install prefix.
Instance directory (/etc/cubicweb.d) and Instance Data directory
(/var/lib/cubiweb/) are now put withing the install prefix of cubicweb.
When this prefix is /usr/ old behaviour are used (they are putted in the root
directory).
This change will make cubicweb much more virtual env friendly.