[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.

# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s entity's classes"""