Do not use openstack clients as a thread safe objects

OpenStack python clients are not thread safe so we are not able
to share them between different processes.

This patches makes next:

* removes __openstack__clients__ and __admin_clients__
  from benchmark.runner module
* pass to benchmark.runner._scenario_run_once dicts with endpoints
* refactors creation of openstack clients from endpoint dict
* refactors benchmark.utils._prepare_for_instance_ssh to work with
  endpoint dicts
* improve unit tests
* skip test_run_scenario_continuously_for_duration cause it produce a lot
  of race conditions in gates

TODO: Finish refactoring of ut

After this patch you will be able to use Rally as a benchmarking
tool=)

Change-Id: Ifa1e0e55516ac1ca5117203811c375b2c6aff0d7
Closes-Bug: #1282145