pithos: Make pithos.conf.py Synnefo wide accessible

Rename pithos.conf.py to gunicorn-archipelago.py and move it
to snf-webproject under the directory gunicorn-hooks to
make it wide accessible.
snf-deploy: Update Gunicorn conf file

Update Gunicorn conf file and relevant files.
pithos: Remove obsolete Gunicorn worker
docs: Update upgrade doc for 0.16
docs: Update Debian installation guide
docs: Update CentOS installation guide