supervisor-xserver: Add `XSERVER_LANG` variable to make `LANG` configurable
supervisor-xserver: Extract `xserver_gunicorn_workers` variable to defaults
supervisor-xserver: Move playbooks to supervisorctl and supervisor.conf
supervisor-xserver: Convert `xserver.conf` to supervisor configuration format
supervisor-xserver: Switch from supervisor reloads to ansible "present" action

`supervisorctl reload` not only reloads the configuration files, but
also restarts all services, which causes all services to be reloaded
several times, and causes errors when the playbook later attempts to
start the services after changes have been applied.
Merge pull request #404 from antoviaque/supervisor-xserver

Move xserver to supervisor