Remove unnecessary DB call to find EC2 AZs.

Update get_availability_zone_by_host, so it doesn't need a list of services,
and remove unneeded service_get_all_by_host calls. This removes N DB calls in
the EC2 api when doing a DescribeInstances, where N is number if instances
that are being described.

Partially fix bug 1157437

Change-Id: I0050ba736828faad94549bc6e55f4ca1d05dcbe7
(cherry picked from commit 7ab98e935a)

nova/api/ec2/cloud.py

@@ -1178,8 +1178,7 @@ class CloudController(object):
             self._format_instance_bdm(context, instance['uuid'],
                                       i['rootDeviceName'], i)
             host = instance['host']
-            services = db.service_get_all_by_host(context.elevated(), host)
-            zone = ec2utils.get_availability_zone_by_host(services, host)
+            zone = ec2utils.get_availability_zone_by_host(host)
             i['placement'] = {'availabilityZone': zone}
             if instance['reservation_id'] not in reservations:
                 r = {}

nova/api/ec2/ec2utils.py

@@ -115,11 +115,9 @@ def get_ip_info_for_instance(context, instance):
     return get_ip_info_for_instance_from_nw_info(nw_info)
 
 
-def get_availability_zone_by_host(services, host, conductor_api=None):
-    if len(services) > 0:
-        return availability_zones.get_host_availability_zone(
-            context.get_admin_context(), host, conductor_api)
-    return 'unknown zone'
+def get_availability_zone_by_host(host, conductor_api=None):
+    return availability_zones.get_host_availability_zone(
+        context.get_admin_context(), host, conductor_api)
 
 
 def id_to_ec2_id(instance_id, template='i-%08x'):

nova/api/metadata/base.py

@@ -106,10 +106,8 @@ class InstanceMetadata():
         ctxt = context.get_admin_context()
 
         capi = self.conductor_api
-        services = capi.service_get_all_by_host(ctxt.elevated(),
-                                                instance['host'])
         self.availability_zone = ec2utils.get_availability_zone_by_host(
-                services, instance['host'], capi)
+                instance['host'], capi)
 
         self.ip_info = ec2utils.get_ip_info_for_instance(ctxt, instance)