Expand config validation logic and tests

[cfunkhouser]

This change adds an extra check for provider validity at config parse time, and expands the tests. There are no multi-provider tests in this change.

[asjoyner]

as per table-side discussion, please do away with the validProviders string slice, and implement the awesome register provider and associated magic.

[cfunkhouser]

Okay, I've implemented and tested the drive client provider registration logic we discussed around the table.

This gets us some more complexity in the drive module; namely:

• A RegisterProvider function, which is meant to be called from the module init() function
• A ValidProvider function which checks if a provider is registered
• A NewClient function - which has the same signature as the per-provider constructor - and creates the new client of the correct provider type based on the Provider member in the Config struct

I've also updated the drive provider modules, and shadeutil.