Add code to enable single-value response for http clients

This commit adds the RestBody class as well as
one service example of its use.

Partially implements: bp clients-return-one-value

Change-Id: Idf3a07dca8c55f6b75645b7ce8ead5e66fb2aff4
Merge "Add code to enable single-value response for http clients"
Merge branch 'master' of https://github.com/openstack/tempest