Pass around Roles instead of Strings

Avoid looking up roles by names everywhere. This avoids the awkward
role/role_config naming as well.

lib/kamal/configuration.rb

@@ -92,7 +92,19 @@ class Kamal::Configuration
   end
 
   def primary_host
-    role(primary_role)&.primary_host
+    primary_role&.primary_host
+  end
+
+  def primary_role_name
+    raw_config.primary_role || "web"
+  end
+
+  def primary_role
+    role(primary_role_name)
+  end
+
+  def allow_empty_roles?
+    raw_config.allow_empty_roles
   end
 
   def traefik_roles
@@ -212,14 +224,6 @@ class Kamal::Configuration
     raw_config.asset_path
   end
 
-  def primary_role
-    raw_config.primary_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 && ensure_retain_containers_valid && ensure_valid_service_name
@@ -268,12 +272,12 @@ 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
 
-      unless role_names.include?(primary_role)
-        raise ArgumentError, "The primary_role #{primary_role} isn't defined"
+      unless role_names.include?(primary_role_name)
+        raise ArgumentError, "The primary_role #{primary_role_name} isn't defined"
       end
 
-      if role(primary_role).hosts.empty?
-        raise ArgumentError, "No servers specified for the #{primary_role} primary_role"
+      if primary_role.hosts.empty?
+        raise ArgumentError, "No servers specified for the #{primary_role.name} primary_role"
       end
 
       unless allow_empty_roles?