Optimize the instance fetched by floating_ips API

The floating_ips API fetches an instance without flavor information
and then passes it to the Network API, which eventually calls
_get_instrance_nw_info() which requires access to the flavor. This
triggers a lazy load in 100% of the situations, which is really not
necessary and wasteful.

This patch adds flavor to the list of pre-queried attributes on the
instances fetched by this API to help reduce that waste.

Change-Id: I15eae4e8c03739a34cef9c90989ad0615a96a963
Partial-Bug: #1540526

nova/api/openstack/compute/floating_ips.py

@@ -71,7 +71,8 @@ def get_instance_by_floating_ip_addr(self, context, address):
         raise webob.exc.HTTPConflict(explanation=ex.format_message())
 
     if instance_id:
-        return common.get_instance(self.compute_api, context, instance_id)
+        return common.get_instance(self.compute_api, context, instance_id,
+                                   expected_attrs=['flavor'])
 
 
 def disassociate_floating_ip(self, context, instance, address):
@@ -193,7 +194,8 @@ def _add_floating_ip(self, req, id, body):
 
         address = body['addFloatingIp']['address']
 
-        instance = common.get_instance(self.compute_api, context, id)
+        instance = common.get_instance(self.compute_api, context, id,
+                                       expected_attrs=['flavor'])
         cached_nwinfo = compute_utils.get_nw_info_for_instance(instance)
         if not cached_nwinfo:
             LOG.warning(