Adds missing entry in setup.cfg for V3 API shelve plugin

Adds the missing entry in setup.cfg which allows the V3 API
shelve plugin to be loaded. Also adds a v3 version of the unittests
for the shelve plugin.

Change-Id: I737ca0cae503b726b0103a4ae169ffca91188c87
Closes-Bug: #1233454
(cherry picked from commit 96895e29b1a1387c185fe740021bb636aa02709f)
Merge "Adds missing entry in setup.cfg for V3 API shelve plugin" into milestone-proposed