Add Sahara client for scenarios tests

Due to the very high load during the real life scenarios tests for
Sahara we'd like to be able to use explicitly stored scenarios tests at the
beginning.

There is a plan to add some non compute-intensive scenarios for Sahara:

* node group / cluster templates workflows;
* integration between Sahara and Swift;
* cluster provisioning tests using fake plugins to work with cirros
images.

Change-Id: I56f5b1c6528fd406f1e1b3ffef79a85aeb2ac144
Merge "Add Sahara client for scenarios tests"