All role specific proxy configuration

By default only the primary role runs the proxy. To disable the proxy
for that role, you can set `proxy: false` under it.

For other roles they default to not running the proxy, but you can
enable it by setting `proxy: true` for the role, or alternatively
setting a proxy configuration.

The proxy configuration will be merged into the root proxy configuration.

lib/kamal/configuration/validator/proxy.rb

@@ -1,9 +1,11 @@
 class Kamal::Configuration::Validator::Proxy < Kamal::Configuration::Validator
   def validate!
-    super
+    unless config.nil?
+      super
 
-    if config["host"].blank? && config["ssl"]
-      error "Must set a host to enable automatic SSL"
+      if config["host"].blank? && config["ssl"]
+        error "Must set a host to enable automatic SSL"
+      end
     end
   end
 end