Add missing _ imports to allow doc generation

Fixes bug: 1209413

Change-Id: I7d405ceb6a500cbb3d0eb19e85cb0540564e0643

heat/common/config.py

@@ -26,6 +26,7 @@ from oslo.config import cfg
 
 from heat.common import wsgi
 
+from heat.openstack.common.gettextutils import _
 from heat.openstack.common import log as logging
 from heat.openstack.common import rpc
 

heat/common/exception.py

@@ -21,6 +21,7 @@ import functools
 import urlparse
 import sys
 from heat.openstack.common import exception
+from heat.openstack.common.gettextutils import _
 
 
 OpenstackException = exception.OpenstackException

heat/common/wsgi.py

@@ -45,6 +45,7 @@ import webob.exc
 
 from heat.common import exception
 from heat.openstack.common import gettextutils
+from heat.openstack.common.gettextutils import _
 from heat.openstack.common import importutils