Change assertTrue(isinstance()) by optimal assert

assertTrue(isinstance(A, B)) or assertEqual(type(A), B) in tests
should be replaced by assertIsInstance(A, B) provided by testtools.

I have searched all the tests, there is only one wrong usage.

Change-Id: Ib1db1a2dca7b5d8cbfe823973e4b571d0f0925c5
Closes-bug: #1268480
Set purge-props header correctly in image update

Currently when an image is updated, the purge property
header is only set to true in some cases, but when
required it isn't set to false

Change-Id: I885a82643d2620f393f21c36b3ad95cb7ed43f2c
Closes-Bug: 1318079
Prepend '/' to the delete url for the v2 client

... and update tests to match.

The missing slash results in a non-absolute DELETE request
being sent to the API.
E.g.
DELETE v2/images/62fac489-23b4-4929-87af-2e7236e8542b HTTP/1.1

This is not strictly valid http/1.1 - rfc2616 specifies that the path must
be absolute.
This doesn't cause a problem for the API server, but this can cause
problems if the API server is fronted by something else (see #133161).

It also means that the curl command logged in debug mode has a
bad url.
E.g.
curl -i -X DELETE ... http://10.0.0.13:9292v2/images/...

Change-Id: Ib0c749dedbfcf07303fcddae4512db61b0f3fd78
Closes-bug: #1327101
Merge "Change assertTrue(isinstance()) by optimal assert"
Merge "Prepend '/' to the delete url for the v2 client"
Merge "Set purge-props header correctly in image update"
Add the six module dependency

Change-Id: Ie57c4e8bad2934acb5217a88713099374c20dde9
Closes-Bug: #1333872