It's now possible to do: `just install-service prometheus -l host.example.com`

It should be noted that this cannot be used for the initial install
of services which require a database or have other dependencies.
Those would typically need to invoke the playbook with
`--tags=install-postgres,install-SERVICE`, etc.

The purpose of this shortcut is to easily rebuild and restart
a single serice subsequently. For those cases, often times there's no need to
reinitialize the database and other components and simply running a
single component's tasks is enough.

We keep the Makefile for now, but don't mention it to new
users anymore.