availability_zone extension missing from API ref

[Boden R]

The availability_zone extension is not documented in the API ref:
- The availability_zones resource is not documented.
- The az extension is not documented on the agent api-ref nor is the availability_zone attribute added to agents.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/566154

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/566184

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/566184
Committed: https://git.openstack.org/cgit/openstack/neutron-lib/commit/?id=29609c6aa84a0acfa44698ea889f1790a6a9bf92
Submitter: Zuul
Branch: master

commit 29609c6aa84a0acfa44698ea889f1790a6a9bf92
Author: Hongbin Lu <email address hidden>
Date: Thu May 3 23:04:34 2018 +0000

    api-ref: add availability_zone extension

    Depends-On: Id882f949cc73a34290c311f3ce3d69d1b809c29f
    Change-Id: Icbf427f20ca912a40d68e8042abeeabffeb3005f
    Closes-Bug: #1733362

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/neutron-lib 1.19.0 release.

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: master
Review: https://review.opendev.org/763246