Replaces methods deprecated in pymongo3.0

This is a cherry-pick from f0598b213dd45b8d5dcd1a773b2022104bf8dad3 in
Ceilometer.

Change-Id: I6e4bf806537fda3002d7c9a084e364ecb5f4303c

aodh/storage/mongo/utils.py

@@ -90,13 +90,11 @@ class ConnectionPool(object):
     @staticmethod
     def _mongo_connect(url):
         try:
-            if cfg.CONF.database.mongodb_replica_set:
-                client = MongoProxy(
-                    pymongo.MongoReplicaSetClient(
-                        url,
-                        replicaSet=cfg.CONF.database.mongodb_replica_set))
-            else:
-                client = MongoProxy(pymongo.MongoClient(url))
+            client = MongoProxy(
+                pymongo.MongoClient(
+                    url, replicaSet=cfg.CONF.database.mongodb_replica_set
+                )
+            )
             return client
         except pymongo.errors.ConnectionFailure as e:
             LOG.warn(_('Unable to connect to the database server: '