Add shard support to the client

The patch adds Shard support of the client. This will allow users to
create and delete shards from Marconi. The API requires the Marconi
instance to be and admin instance. However, there are no checks,
restrictions, yet.

This patch doesn't implement Shards listing. As soon as the
MessageIterator lands, it'll be possible to reuse that code for the
ShardsIterator.

Partially-Implements blueprint: shard-management
Partially-Implements blueprint: python-marconiclient-v1

Change-Id: I932c98d68a4a81b710dbea75a6bac123b7afaa08
Merge "Add shard support to the client"