debian python-boto package doesn't ship the "bins" so fetch_file and s3put are not there. Write quick and dirty versions of them. Then fetch the last database from s3 and restore it. add an hourly cron job to dump the database and put to s3.

server.yaml

#cloud-config
runcmd:
 - |
    echo "#!/usr/bin/python
    import boto, sys
    boto.connect_s3().get_bucket(sys.argv[0]).get_key(sys.argv[1]).get_contents_to_filename(sys.argv[2])
    ">/usr/bin/s3get
 - chmod +x /usr/bin/s3get
 - s3get bucket db_dumps/$HOSTNAME-dbname.pg_dump /var/tmp/dbname.pg_dump
 - su -c 'pg_restore -d dbname /var/tmp/dbname.pg_dump' postgres
 - |
    echo "#!/usr/bin/python
    import boto, sys
    boto.connect_s3().get_bucket(sys.argv[0]).new_key(sys.argv[2]).set_contents_from_filename(sys.argv[1])
    ">/usr/bin/s3put
 - chmod +x /usr/bin/s3put
 - echo '0 * * * * postgres (cd /var/lib/postgresql; pg_dump -Fc dbname > $HOSTNAME-dbname.pg_dump; s3put bucket $HOSTNAME-dbname.pg_dump db_dumps/$HOSTNAME-dbname.pg_dump' > /etc/cron.d/db_dump