Fix for registration command packages (#1225) (#1242)

it is a space separated string not a list
https://apidocs.theforeman.org/foreman/latest/apidoc/v2/registration_commands/create.html

(cherry picked from commit 4f7c162)

Co-authored-by: dosas <dosas@users.noreply.github.com>

nailgun/entities.py

@@ -6522,7 +6522,7 @@ def __init__(self, server_config=None, **kwargs):
             'jwt_expiration': entity_fields.IntegerField(default=4),
             'repo': entity_fields.StringField(default=''),
             'repo_gpg_key_url': entity_fields.URLField(default=''),
-            'packages': entity_fields.ListField(default=[]),
+            'packages': entity_fields.StringField(),
             'update_packages': entity_fields.BooleanField(default=False),
             'force': entity_fields.BooleanField(default=False),
             'ignore_subman_errors': entity_fields.BooleanField(default=False),