Blueprint-flagged-module-loading

We currently use utils.LazyPluggable and utils.import_class to load classes. We should use a single method. I suggest we use the techniques of LazyPluggable, but modify it so we don't need to specify a list of backends, but can give a class reference like those used in import_class. This way we get the lazy-loading of the backend, so that we can be less concerned with order of operations such as loading flags, and we don't need to maintain a list of possible values and can even reverence classes in other packages and allow drivers to be written outside of the main nova source tree.