Update upstart.conf

Instead of having the sysadmin edit the `/etc/init/flynn-host.conf` file during installation, source the `ETCD_DISCOVERY` variable from a single-line `/etc/default/etcd-discovery` file.

The `/etc/default/etcd-discovery` file can then be easily generated during host/cluster provisioning with a command like `echo ETCD_DISCOVERY=$(curl -s "https://discovery.etcd.io/new") > /etc/default/etcd-discovery`