Clean imports in code

Most imports that don't match[1], has been cleaned in [2]. There are still
some left.

[1] http://docs.openstack.org/developer/hacking/#imports
[2] I5aa415966a94bec00257c4d34bd5633de7bb34c8

Change-Id: Ib50baf7f7a2da18ffb305c24667dd921a4bcea89

dragonflow/common/utils.py

@@ -15,7 +15,7 @@ import os
 import sys
 import time
 
-from collections import deque
+import collections
 import eventlet
 import greenlet
 from neutron_lib import constants as n_const
@@ -184,7 +184,7 @@ class wrap_func_retry(object):
 class RateLimiter(object):
     def __init__(self, max_rate=3, time_unit=1, **kwargs):
         self.time_unit = time_unit
-        self.deque = deque(maxlen=max_rate)
+        self.deque = collections.deque(maxlen=max_rate)
 
     def __call__(self):
         if self.deque.maxlen == len(self.deque):

dragonflow/db/db_store.py

@@ -13,7 +13,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from collections import defaultdict
+import collections
 import copy
 import threading
 
@@ -73,7 +73,7 @@ class TenantDbStore(object):
 class DbStore(object):
 
     def __init__(self):
-        self.tenant_dbs = defaultdict(TenantDbStore)
+        self.tenant_dbs = collections.defaultdict(TenantDbStore)
         self.networks = {}
 
     def get(self, table_name, key, topic):