Fix AZ List Detail schema to allow hosts as None

List AZ with Details response can return host as None
for non available AZ from nova.

But response schema [1] always expect host as non-None
value which is not true for non available AZ on nova side.

Change-Id: I015ea26eb47ef179d55f3e452e871bfeec406365
Closes-Bug: #1662781

tempest/lib/api_schema/response/compute/v2_1/availability_zone.py

@@ -51,7 +51,7 @@ base = {
 }
 
 detail = {
-    'type': 'object',
+    'type': ['object', 'null'],
     'patternProperties': {
         # NOTE: Here is for a hostname
         '^[a-zA-Z0-9-_.]+$': {