feat(contrib): add PREFIX option to digitalocean script

When provisioning new nodes, they are prefixed with a name of "deis-$i",
where $i is the node ID. Adding a prefix option allows us to prefix
the name with something other than "deis" such that we can discern
between different clusters on the same DO account.
Merge pull request #2630 from bacongobbler/add-do-prefix

feat(contrib): add PREFIX option to digitalocean script