Add number and file validators

 * number: Check a numeric argument specifying min, max. Optionally
           require input to be integer.
 * 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
Merge "Add number and file validators"