SSH proxy: allow using a bare hostname without root@

Otherwise we can't connect to the proxy as the local user and we can't
use ~/.ssh/config to set User directives.

Defaulting to root@ is hard to deprecate without introducing new config.
A clean break is probably clearest.

lib/kamal/configuration/docs/ssh.yml

@@ -29,8 +29,8 @@ ssh:
 
   # Proxy host
   #
-  # Specified in the form <host> or <user>@<host>:
-  proxy: root@proxy-host
+  # Specified in the form <host> or <user>@<host>
+  proxy: proxy-host
 
   # Proxy command
   #

lib/kamal/configuration/ssh.rb

@@ -19,9 +19,9 @@ class Kamal::Configuration::Ssh
   end
 
   def proxy
-    if (proxy = ssh_config["proxy"])
-      Net::SSH::Proxy::Jump.new(proxy.include?("@") ? proxy : "root@#{proxy}")
-    elsif (proxy_command = ssh_config["proxy_command"])
+    if proxy = ssh_config["proxy"]
+      Net::SSH::Proxy::Jump.new(proxy)
+    elsif proxy_command = ssh_config["proxy_command"]
       Net::SSH::Proxy::Command.new(proxy_command)
     end
   end