Explicitly install snapd package

It was not installed on fornax for example

site-cookbooks/kosmos-nginx/resources/nginx_certbot_site.rb

@@ -9,6 +9,8 @@ property :site, String
 action :create do
   return if node.chef_environment == "development"
 
+  package "snapd"
+
   domain = new_resource.domain
   site = new_resource.site || domain
   root_directory = "/var/www/#{domain}"