Add integer and file validators

 * integer_range: Check an argument is an integer within the specified range
 * file_exists: Check that a file exists and can be opened

 These validators are used in the runcommand in instance benchmark
 (patch pending)

Change-Id: Ia12e95b3661c97ae6f4c54caf44fdf33b72aa736
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