Ignore content_view_environment_id in HostGroup read()

This field is write-only — the API response does not include it,
so read() must skip it to avoid a KeyError.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jeremylenz

nailgun/entities.py

@@ -4118,6 +4118,7 @@ def read(self, entity=None, attrs=None, ignore=None, params=None):
         ignore.add('root_pass')
         ignore.add('kickstart_repository')
         ignore.add('compute_resource')
+        ignore.add('content_view_environment_id')
 
         attrs = attrs or self.read_json()
         attrs['parent_id'] = attrs.pop('ancestry')  # either an ID or None