Nova-all: Replace basestring by six for python3 compatability

The builtin basestring abstract type was removed in python3,
use str instead, we need to using six.string_types to replace
the basestring for py3 compatibility.

Partial Implements bp: basestring-py3-compatible

Change-Id: I17e87e49e7ebf3cc9c391e9beace8c45d2d2b689
Merge "Nova-all: Replace basestring by six for python3 compatability"