Python3: use dict.keys() instead of dict.iterkeys()

The "keys" method works on both Python 2 and 3, and the performance
impact should be negligible.

Change-Id: I4771797859666000921e4e38cc5de72a8c084ca0
Blueprint: neutron-python3

neutron/extensions/quotasv2.py

@@ -48,7 +48,7 @@ class QuotaSetsController(wsgi.Controller):
         self._update_extended_attributes = True
 
     def _update_attributes(self):
-        for quota_resource in QUOTAS.resources.iterkeys():
+        for quota_resource in QUOTAS.resources.keys():
             attr_dict = EXTENDED_ATTRIBUTES_2_0[RESOURCE_COLLECTION]
             attr_dict[quota_resource] = {
                 'allow_post': False,

neutron/plugins/sriovnicagent/sriov_nic_agent.py

@@ -309,7 +309,7 @@ class SriovNicAgentConfigParser(object):
         exists in device mappings
         """
         dev_net_set = set(self.device_mappings.values())
-        for dev_name in self.exclude_devices.iterkeys():
+        for dev_name in self.exclude_devices.keys():
             if dev_name not in dev_net_set:
                 raise ValueError(_("Device name %(dev_name)s is missing from "
                                    "physical_device_mappings") % {'dev_name':