do not create the ENV file for config:get

This fixes a bug introduced when deploying apps where the
nginx-vhosts plugin runs ``dokku config:get $APP NO_VHOST``. The initial
run of this actually was creating the ``ENV`` file and dumping to
stdout the value "NO_VHOST=-----> Creating /home/dokku/foo/ENV". This
was obviously no good because nginx-vhosts was considering this to be a
value of true.

I've modified this to simply not return anything when the value does not
exist. A better update may be to output the messages to stderr instead
of stdout.