Add allow_empty_roles to control aborting on roles with no hosts.

This added flexibility allows you to define base roles that might not
necessarily exist in each deploy destination.

lib/kamal/cli/templates/deploy.yml

@@ -91,3 +91,8 @@ registry:
 # Caution: there's no support for role renaming yet, so be careful to cleanup
 #          the previous role on the deployed hosts.
 # primary_web_role: web
+
+# Controls if we abort when see a role with no hosts. Disabling this may be
+# useful for more complex deploy configurations.
+#
+# allow_empty_roles: false

lib/kamal/configuration.rb

@@ -210,6 +210,11 @@ class Kamal::Configuration
     raw_config.primary_web_role || "web"
   end
 
+  def allow_empty_roles?
+    raw_config.allow_empty_roles
+  end
+
+
   def valid?
     ensure_destination_if_required && ensure_required_keys_present && ensure_valid_kamal_version
   end
@@ -257,12 +262,6 @@ class Kamal::Configuration
         raise ArgumentError, "You must specify a password for the registry in config/deploy.yml (or set the ENV variable if that's used)"
       end
 
-      roles.each do |role|
-        if role.hosts.empty?
-          raise ArgumentError, "No servers specified for the #{role.name} role"
-        end
-      end
-
       unless role_names.include?(primary_web_role)
         raise ArgumentError, "The primary_web_role #{primary_web_role} isn't defined"
       end
@@ -271,6 +270,18 @@ class Kamal::Configuration
         raise ArgumentError, "Role #{primary_web_role} needs to have traefik enabled"
       end
 
+      if role(primary_web_role).hosts.empty?
+        raise ArgumentError, "No servers specified for the #{primary_web_role} primary_web_role"
+      end
+
+      unless allow_empty_roles?
+        roles.each do |role|
+          if role.hosts.empty?
+            raise ArgumentError, "No servers specified for the #{role.name} role. You can ignore this with allow_empty_roles: true"
+          end
+        end
+      end
+
       true
     end