Add benchmark tests for glance

The patch adds the following benchmark tests for glance:
* Add and delete image
* Boot the several instances from added image

Also some changes are added related with using
fake Glance client the for following unit tests:
* test_generic_cleanup in tests/benchmark/test_runner.py
* test_snapshot_server in tests/benchmark/scenarios/nova/test_servers.py

Implements: blueprint benchmark-scenarios
Change-Id: I31471a91455bd20eea000a66c59320178455cb50