Unify deploy secret keys for the main server

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.

config/deploy.rb

@@ -3,7 +3,15 @@ lock "~> 3.17.1"
 
 def deploysecret(key)
   @deploy_secrets_yml ||= YAML.load_file("config/deploy-secrets.yml")[fetch(:stage).to_s]
-  @deploy_secrets_yml.fetch(key.to_s, "undefined")
+  @deploy_secrets_yml.fetch(key.to_s, "")
+end
+
+def main_deploy_server
+  if deploysecret(:server1) && !deploysecret(:server1).empty?
+    deploysecret(:server1)
+  else
+    deploysecret(:server)
+  end
 end
 
 set :rails_env, fetch(:stage)