Heat: add availability_zone

Heat: add availability_zone

Users would like to ensure placement of VMs on specific compute nodes so
that the measurements are meaningful. Examples: Measure network
performance in different scenarios (VMs in same host, in different
hosts, across fabric, across tenants)

Example:

context:
  name: yardstick
  placement_groups:
    pgrp1:
      policy: "availability"
  servers:
    tg_0:
      floating_ip: true
      placement: "pgrp1"
      availability_zone: "zone2"
    vnf_0:
      floating_ip: true
      placement: "pgrp1"
      availability_zone: "zone1"

Change-Id: I28a757c25ae3f5b3571ab3edd82d51ceba32c302
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>