We were using `server` on staging but `server1` and `server2` on
preproduction and production.
The reason behind it is we've always used one server on staging but
sometimes we've used several servers on preproduction and production.
However, this is a bit of a mess on installations which have only one
server on preproduction or production and need to use the `server` key
for the staging environments but `server1` for other environments.
So, in order to keep compatibility with existing Consul installations,
we're now allowing either `server` or `server1` on any environment.
8aa113a47c