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 "Prepend '/' to the delete url for the v2 client"