The default start-stop-daemon start function expects the command
variable to be defined to point to the daemon we want to start.
If the variable is undefined, this means that there will be nothing to
start, and in this case we should complain because it is possible that
the script writer made a typo in the variable name.
The original way of doing this allowed users to change the supervisor in
conf.d/*. This changes this so that the supervisor setup can be done in
the service script itself.