Move Mastodon deploy flag to the mastodon role

Set kosmos-mastodon.deploy to false as a role default, making it overridable per node, and drop the node-level attribute.

roles/mastodon.rb

@@ -1,5 +1,9 @@
 name "mastodon"
 
+default_attributes 'kosmos-mastodon' => {
+  'deploy' => false
+}
+
 run_list %w(
   role[postgresql_client]
   kosmos-mastodon::libretranslate