Set import exceptions in tox.ini configuration

Instead of "from i18n import _ 
            from i18n import _LE"
this style "from i18n import _, _LE" is recommended as following:
http://docs.openstack.org/developer/hacking/#real-world-import-order-examples
but the pep8 failed: H301 one import per line
so it needs to set import exceptions in tox.ini to avoid the problem

Change-Id: I2de6cef388af65630dcba031a055f6757695006f

designate/backend/agent.py

@@ -36,8 +36,7 @@ import dns.opcode
 from oslo_config import cfg
 from oslo_log import log as logging
 
-from designate.i18n import _LI
-from designate.i18n import _LW
+from designate.i18n import _LI, _LW
 from designate.backend import base
 from designate import exceptions
 from designate.mdns import rpcapi as mdns_api

designate/objects/adapters/base.py

@@ -19,8 +19,7 @@ import six
 from designate import objects
 from designate import utils
 from designate import exceptions
-from designate.i18n import _LE
-from designate.i18n import _LI
+from designate.i18n import _LE, _LI
 
 LOG = log.getLogger(__name__)
 

tox.ini

@@ -131,3 +131,4 @@ exclude = .venv,.git,.tox,dist,doc,*lib/python*,*egg,build,tools,.ropeproject
 
 [hacking]
 local-check-factory = designate.hacking.checks.factory
+import_exceptions = designate.i18n