cf v3 commands don't appear to support commands in manifest files. They
say that they do, but in practice they fail on cf v3-zdt-push with an
error message "No process types returned from stager". This can be
solved by moving the command from the manifest to a Procfile.
However, the Procfile is part of the source code, and as such is the
same for each app. To get around this, make the Procfile command invoke
a new wrapper script, which checks the NOTIFY_APP_NAME env var and then
calls the correct command
