Reduce the query time of the instances when call get_instance_list()

The problem is that watcher is passing limit=-1 to novaclient when
listing servers which will always make at least two API calls to be
sure it's done paging:

https://github.com/openstack/python-novaclient/blob/13.0.1/novaclient/v2/servers.py#L896

If we can determine before we list servers that there are only a
certain number where the number of servers is less than 1000. For
example: 4, we should just pass the limit=len(servers) to novaclient
and avoid the second call for paging which takes extra time and
yields no results.

Change-Id: I797ad934a0f8496dbcbf65798e28b0443f238137
Closes-Bug: #1834679

watcher/decision_engine/model/collector/nova.py

@@ -324,8 +324,18 @@ class ModelBuilder(object):
             return
         host = node.service["host"]
         compute_node = self.model.get_node_by_uuid(host)
+        filters = {'host': host}
+        limit = len(instances) if len(instances) <= 1000 else -1
         # Get all servers on this compute host.
-        instances = self.nova_helper.get_instance_list({'host': host})
+        # Note that the advantage of passing the limit parameter is
+        # that it can speed up the call time of novaclient. 1000 is
+        # the default maximum number of return servers provided by
+        # compute API. If we need to request more than 1000 servers,
+        # we can set limit=-1. For details, please see:
+        # https://bugs.launchpad.net/watcher/+bug/1834679
+        instances = self.nova_helper.get_instance_list(
+            filters=filters,
+            limit=limit)
         for inst in instances:
             # Add Node
             instance = self._build_instance_node(inst)