Use six.string_types instead of basestring

basestring only exists in Python 2. Use six.string_types to make code
Python 3 compatible. See http://pythonhosted.org/six/#six.string_types

Part of blueprint keystone-py3kcompat

Change-Id: I5022259cfd648ba2680aabbb0fbc403f10c5dc7e
Merge "Use six.string_types instead of basestring"