Use six.move.xrange to replace xrange

in python3.x, there is no method named xrange(),
it has been replaced by range(),which is equal to xrange()
in python2.x. so we must fix this issue.
we use six module to fix this issue.

Change-Id: Ia901798cf16b9a48982e6845fa3e5deffc3bd509
Closes-Bug: #1268439

heat/common/short_id.py

@@ -19,6 +19,8 @@ each comprise 12 (lower-case) alphanumeric characters.
 import base64
 import uuid
 
+from six.moves import xrange
+
 
 def _to_byte_string(value, num_bits):
     '''

heat/engine/dependencies.py

@@ -15,6 +15,8 @@
 import collections
 import itertools
 
+from six.moves import xrange
+
 from heat.common import exception
 
 from heat.openstack.common.gettextutils import _

heat/engine/resources/random_string.py

@@ -15,6 +15,8 @@
 import random
 import string
 
+from six.moves import xrange
+
 from heat.db import api as db_api
 from heat.engine import constraints
 from heat.engine import properties

heat/tests/test_parser.py

@@ -18,6 +18,7 @@ import time
 from keystoneclient import exceptions as kc_exceptions
 from mox import IgnoreArg
 from oslo.config import cfg
+from six.moves import xrange
 
 from heat.engine import environment
 from heat.common import exception