Document 'availability_zone' in API reference

[hongbin]

Recently, we added support for running container in specific availability zone:
1. https://review.openstack.org/#/c/543255/
2. https://review.openstack.org/#/c/543145/

The first patch adds a new parameter 'availability_zone' on the request of POST /containers endpoint. The second patch adds 'availability_zone' to the response of GET /services endpoint. We need to document these in the API reference.

NOTE:
* The API reference can be found in: https://developer.openstack.org/api-ref/application-container/ .
* The source of API reference can be found in: https://github.com/openstack/zun/tree/master/api-ref/source

[hongbin (hongbin034)]

Hi @Madhuri,

Sorry, I have already uploaded a patch for this: https://review.openstack.org/#/c/558346/ .