docs: Update centos install guide wrt synnefo user

Additionally change the NFS dirs to /srv/pithos/{blocks,maps,locks}.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
docs: Update debian install guide wrt synnefo user

In short Synnefo will run as synnefo:www-data while Archipelago as
synnefo:synnefo. The exported directories in NFS should be
root:synnefo with g+ws.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
docs: Update admin guide wrt synnefo user

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
docs: Update upgrade notes wrt synnefo user

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