Add ObjectStorageClient for cleanup

In object storage clients, there is a lot of duplicated code for
setting CONF. This patch adds ObjectStorageClient for removing them.

Change-Id: I07eb4af34b20ae94a09b6fa90ffeb3bc19e14762
Merge network clients to a single class

There are two network clients as a base one and a JSON one. This
implementation way is different from the other projects' clients.
Now we need to implement a rest client method as a library function,
and it is necessary to make the rest implementations consistent now.
Then, this patch merges network clients to a single class like the
other projects' ones.

Change-Id: I8373f372dcc7b93c185bb5fc67f6a84429505672
Merge "Add ObjectStorageClient for cleanup"
Merge "Merge network clients to a single class"
Merge branch 'master' of https://github.com/openstack/tempest