Setup direct monitoring of digital ocean VPSs

[hellais]

In order to do this, we need some kind of way of doing direct addressing of the individual test helper digital ocean hosts and have a functioning TLS setup there. We can either do this by doing some routing at the nginx layer by hostname on the front host or setting up a specific zone that we delegate to digital ocean so that cert renewal can be handled independently (something like do.prod.ooni.io)

[hellais]

This is done as part of: #96