Extract getting instance's AZ into a helper module.

An instance's availability zone logic may used by many modules,
so move it from extend API into helper module `nova.availability_zones`.
Also add some tests for the new method.

part of bp: different-availability-zone-filter

Change-Id: I5916a0b09d5dddec338e8e36503953720dfd6bcd

nova/availability_zones.py

@@ -18,6 +18,12 @@
 from oslo.config import cfg
 
 from nova import db
+from nova.openstack.common import memorycache
+
+# NOTE(vish): azs don't change that often, so cache them for an hour to
+#             avoid hitting the db multiple times on every request.
+AZ_CACHE_SECONDS = 60 * 60
+MC = memorycache.get_client()
 
 availability_zone_opts = [
     cfg.StrOpt('internal_service_availability_zone',
@@ -81,3 +87,18 @@ def get_availability_zones(context):
         if zone not in not_available_zones:
             not_available_zones.append(zone)
     return (available_zones, not_available_zones)
+
+
+def get_instance_availability_zone(context, instance):
+    """Return availability zone of specified instance."""
+    host = str(instance.get('host'))
+    if not host:
+        return None
+
+    cache_key = "azcache-%s" % host
+    az = MC.get(cache_key)
+    if not az:
+        elevated = context.elevated()
+        az = get_host_availability_zone(elevated, host)
+        MC.set(cache_key, az, AZ_CACHE_SECONDS)
+    return az