add availability_zone support

this fixing add a availability_zone parameter,
and user could select availability zone as they want.
example .kitchen.yml:

driver:
  name: openstack
  openstack_auth_url: xxxxxxxxxxxxxx
  openstack_username: xxxxxxxxx
  openstack_api_key: xxxxxx
  image_ref: xxxxxxx
  flavor_ref: 1
  server_name: testserver01
  availability_zone: ZONENAME