Use temporary SSH keys

Enable the use of user-associated, temporary SSH keys in place of default SSH key

Work done:

• merge service celery into service `web'
• switch from default id_rsa file to one id_rsa_??? file per user
• delete the id_rsa_??? file in web as well as the associated entry in authorized_keys in worker upon user logout
• use a worker container as remote host
• redeploy.sh script to make it easier to restart with a fresh app