debian: Use dpkg-statoverride for /etc/synnefo/*

Let snf-common handle /etc/synnefo directory. Add setgid (g+s)
permission on /etc/synnefo so that every new file created there
(e.g. local.conf) is readable by the synnefo group (and gunicorn
eventually).

All packages, in their postinst script, will find their conf files
under /etc/synnefo and make use of dpkg-statoverride to chown them to
root:synnefo with 0640 permissions.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
debian: dpkg-statoverride --remove in postrm

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
debian: Let dispatcher run as synnefo:synnefo

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>