Refactoring for generic benchmark cleanup

Initial refactoring which will be used in the generic benchmark cleanup
implementation. Specifically the utility functions which existed in the
NovaScenarios class are now exposed in benchmark.test_utils. Moreover a
few of these functions have been enahanced to be more generic/flexible
in nature.

Subsequent patch will provide the actual generic cleanup.

Implements partial: blueprint benchmark-cloud-cleanup

Change-Id: I7e25dd006e46bf66fd9419090ea310629ba7d19f