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) to be readable by synnefo group (and gunicorn
eventually).

All packages, in their postinst script, will find their conf files
under /etc/synnefo and make use dpkg-statoverride to chown it to
root:synnefo with 0740 permissions.

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

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