Modify assert statement when comparing with None

Replace assertEqual(None, *) with assertIsNone in glance's
tests to have more clear messages in case of failure.

Also, replace assertTrue(* is not None) with assertIsNotNone
for the same reason.

Change-Id: If41a71bd750057c7136b03bef94c04517092134c
Closes-Bug: #1280522
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"
Merge "Modify assert statement when comparing with None"