Add VM Provider factory

To be able to easy extend VM Providers, and use multiple VMProviders
in the same time

So to extend VM provider by your own you have to do couple of simple things

Add new subclass of rally.vmprovider.VMProviderFactory to any modules in
rally.vmprovider.VMProvider.providers.some_module, and implement 4 next methods:

1) upload_image()
2) destroy_image()
3) create_vms()
4) destroy_vms()