Add validations for host/ssl roles

Roles with SSL can only have one server.
Two roles with SSL can't use the same host.

lib/kamal/configuration/proxy.rb

@@ -22,6 +22,10 @@ class Kamal::Configuration::Proxy
     proxy_config.fetch("ssl", false)
   end
 
+  def host
+    proxy_config["host"]
+  end
+
   def deploy_options
     {
       host: proxy_config["host"],

lib/kamal/configuration/role.rb

@@ -75,6 +75,10 @@ class Kamal::Configuration::Role
     @running_proxy
   end
 
+  def ssl?
+    running_proxy? && proxy.ssl?
+  end
+
   def stop_args
     # When deploying with the proxy, kamal-proxy will drain request before returning so we don't need to wait.
     timeout = running_proxy? ? nil : config.drain_timeout
@@ -145,6 +149,12 @@ class Kamal::Configuration::Role
     File.join config.assets_directory, "volumes", [ name, version ].join("-")
   end
 
+  def ensure_one_host_for_ssl
+    if running_proxy? && proxy.ssl? && hosts.size > 1
+      raise Kamal::ConfigurationError, "SSL is only supported on a single server, found #{hosts.size} servers for role #{name}"
+    end
+  end
+
   private
     def initialize_specialized_proxy
       proxy_specializations = specializations["proxy"]