ansible/node: Switch group names from <type>-nodes to <type>_nodes

ansible/node: Switch group names from <type>-nodes to <type>_nodes

This warning:

  [DEPRECATION WARNING]: The TRANSFORM_INVALID_GROUP_CHARS settings is set to
  allow bad characters in group names by default, this will change, but still be
  user configurable on deprecation. This feature will be removed in version 2.10.
   Deprecation warnings can be disabled by setting deprecation_warnings=False in
  ansible.cfg.

According to

  https://github.com/ansible/ansible/issues/56930#issuecomment-498659628

'-' in a group name is invalid and '_' is valid.

Signed-off-by: Martin Schwenke <martin@meltin.net>