Tests object_storage extension listing

This patch adds the feature to list extensions to the object_storage
account_client and then adds one test to validates it works.

This is a first step in view of adding this feature to the config
validation script.

Partially implements config-verification

Change-Id: I8ff31a761d82ea4b7b09307ae1d51108d7f6911e
Merge "Tests object_storage extension listing"