smoke test for backup restoration
replace json.Marshal /w use of json.NewEncoder

this also reduces the amount of code to write when serializing error
messages to the client
introduce the restorer interface to take care of backup restoration
ansible configuration can be passed using CLI arguments

- backup.config - path to backup.json (created by provisioning)
- ansible.playbook - path to ansible-playbook executable
- provisioning.directory - path to ansible provisioning