node_availability_zone deprecated

Updated node_availability_zone to default_availability_zone, as it has been
renamed.
This change was introduced in https://review.openstack.org/#/c/18675/

The docs have been updated according to

https://bugs.launchpad.net/openstack-manuals/+bug/1107419

Change-Id: I8974d96fc34a4a7e8cf6b51468395dc59a80de2c