debian: Let snf-common add synnefo user and group

Add a postinst script in snf-common that creates synnefo
user and group.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
debian: Let snf-common create /var/log/synnefo

..and remove postinst script and dirs from snf-webproject.

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

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
debian: Let /etc/synnefo have setgid flag set

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