Push driver config during Init

Currently the driver configuration is pushed through a separate
api. This makes driver configuration possible at any arbitrary
time. This unncessarily complicates the driver implementation.
More importantly the driver does not get access to it's
configuration before it can do the handshake with libnetwork.
This make the internal drivers a little bit different to
external plugins which can get their configuration before the handshake
with libnetwork.

This PR attempts to fix that mismatch between internal drivers and
external plugins.

Signed-off-by: Jana Radhakrishnan <mrjana@docker.com>

drivers_linux.go

@@ -1,25 +1,19 @@
 package libnetwork
 
 import (
-	"github.com/docker/libnetwork/driverapi"
 	"github.com/docker/libnetwork/drivers/bridge"
 	"github.com/docker/libnetwork/drivers/host"
 	"github.com/docker/libnetwork/drivers/null"
-	o "github.com/docker/libnetwork/drivers/overlay"
+	"github.com/docker/libnetwork/drivers/overlay"
 	"github.com/docker/libnetwork/drivers/remote"
 )
 
-func initDrivers(dc driverapi.DriverCallback) error {
-	for _, fn := range [](func(driverapi.DriverCallback) error){
-		bridge.Init,
-		host.Init,
-		null.Init,
-		remote.Init,
-		o.Init,
-	} {
-		if err := fn(dc); err != nil {
-			return err
-		}
+func getInitializers() []initializer {
+	return []initializer{
+		{bridge.Init, "bridge"},
+		{host.Init, "host"},
+		{null.Init, "null"},
+		{remote.Init, "remote"},
+		{overlay.Init, "overlay"},
 	}
-	return nil
 }