remove users/$user/is_admin metadata, directly write sudo_commands instead

bundles/vmhost/metadata.py

@@ -52,7 +52,7 @@ if node.has_bundle('arch-with-gui'):
 def libvirt_group_for_admins(metadata):
     result = {}
     for user, config in metadata.get('users', {}).items():
-        if config.get('is_admin', False):
+        if 'ALL' in config.get('sudo_commands', set()):
             result[user] = {
                 'groups': {
                     'libvirt',