[202111][sonic-yang] fix the feature state type (#9587) (#9778)

- Why I did it
The feature state can be a jinja template, like in this file - https://github.com/Azure/sonic-buildimage/blob/master/files/build_templates/init_cfg.json.j2#L39.
Without this change it is not possible to validate a configuration file.

- How I did it
Relaxes the constraint on feature state. Feature state leaf can be any string.

- How to verify it
Run UT.

Signed-off-by: Stepan Blyschak <stepanb@nvidia.com>