Glance 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.

Change-Id: Icf709b3499ff7f1ea935045153eb9a5ecb4618ee
Merge "Glance all: Replace basestring by six for python3 compatability"