Correct availability_zone to be non-mandatory in heat

As per CLI and GUI, availability_zone is not mandatory.  In templates,
it is.  This is incorrect.

This patch corrects behaviour to match CLI and GUI.

Change-Id: Ice37340f084ea65bd4b46562145e82ede6bc0df5
Story: 2006586
(cherry picked from commit 2799a5fcd76639a202092ea1b6fc242a8886dc56)

heat/engine/resources/openstack/nova/host_aggregate.py

@@ -56,7 +56,6 @@ class HostAggregate(resource.Resource):
         AVAILABILITY_ZONE: properties.Schema(
             properties.Schema.STRING,
             _('Name for the availability zone.'),
-            required=True,
             update_allowed=True,
         ),
         HOSTS: properties.Schema(

releasenotes/notes/correct-availability-zone-to-be-non-mandatory-b01192007d5e0386.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Erroneously, availability_zone for host aggregate resource types was
+    considered mandatory in heat templates.
+
+    Behaviour has been adjusted to bring this in line with the CLI and GUI, in
+    which it is optional.