Add benchmark for tempest. Part 1

This patch provides bases for tempest benchmarking:

1) Modified rally verification for launching tempest scenarios from benchmark
- moved to usage subunit as a test launcher;
- changed access to several methods;
- changed logging.

2) Added TempestContext, which checks tempest installation, initialization
of testrepository and existence of configuration file for tempest.
Also provides launcher for cleanup cloud after benchmarking.

3) Validator for tempest scenario names.

4) TempestScenario with launcher for single tempest test.

bp benchmark-scenarios-based-on-tempest

Change-Id: If67970a49674750b88d27e77c06d89b4e54d5337