libnetwork: stop passing config to drivers which ignore the config argument
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- Relates to https://github.com/moby/swarmkit/pull/3138

**- What I did**
Removed the config argument from the network driver `Register` functions which ignore the argument value.

**- How I did it**
By refactoring how the libnetwork controller registers network drivers so that the driver registration functions are abstracted away from the controller and therefore no longer need to conform to a single common interface.

**- How to verify it**

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libnetwork/drivers_unsupported.go
//go:build !freebsd && !linux && !windows

package libnetwork

func getInitializers() []initializer {
	return nil
}

//go:build !freebsd && !linux && !windows

package libnetwork

func registerNetworkDrivers(r driverapi.Registerer, driverConfig func(string) map[string]interface{}) error {
	return nil
}

Not a blocker; I guess we should start using `any` in place of `interface{}` now (still getting used to that myself).