Don't hard code the salt loaders base name.

When another software is using salt and specially salt's loader and it defines it's own loader modules, those modules always get prefixed by `salt.loader.LOADED_BASE_NAME`. This change allows those external libs to provide their own value for `LOADED_BASE_NAME` as a `kwarg` to `_create_loader()`.
The internal vs external module type checking should also be configurable.
`loaded_base_name` and `mod_type_check` default to `None` and in that case, to what's defined for salt.
Merge pull request #4530 from s0undt3ch/develop

Don't hard code the salt loaders base name.