In short make gunicorn and snf-dispatcher run as
synnefo:synnefo, and Archipelago as archipelago:synnefo.

This way the synnefo components that run as synnefo:synnefo
(Cyclades, Pithos, etc.) can access the backing storage only through
Archipelago (i.e. named pipes in /dev/shm/posixfd/) and not
directly.

Since we are using NFS we let archipelago user and synnefo group
with common uid and gid respectively across all nodes. The
Archipelago dir to be exported will be owned by archipelago:synnefo
and have group write permissions.

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