Create vm benchmark module

Move the run command in instance benchmark out of the nova module into
its own module (vm). Functionality is then split into a scenario module
and a utils module, following the structure of the other benchmarks.

Tests are also refactored.

Change-Id: Ida26b5047b8e76f72e34c2aab9b8ab94f46bbad6
Merge "Create vm benchmark module"