config: make config file system pluggable

If you are using rclone a library you can decide to use the rclone
config file system or not by calling

    configfile.LoadConfig(ctx)

If you don't you will need to set `config.Data` to an implementation
of `config.Storage`.

Other changes
- change interface of config.FileGet to remove unused default
- remove MustValue from config.Storage interface
- change GetValue to return string or bool like elsewhere in rclone
- implement a default config file system which panics with helpful error
- implement getWithDefault to replace the removed MustValue
- don't embed goconfig.ConfigFile so we can change the methods

backend/alias/alias_internal_test.go

@@ -11,6 +11,7 @@ import (
 	_ "github.com/rclone/rclone/backend/local" // pull in test backend
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/config"
+	"github.com/rclone/rclone/fs/config/configfile"
 	"github.com/stretchr/testify/require"
 )
 
@@ -19,7 +20,7 @@ var (
 )
 
 func prepare(t *testing.T, root string) {
-	config.LoadConfig(context.Background())
+	configfile.LoadConfig(context.Background())
 
 	// Configure the remote
 	config.FileSet(remoteName, "type", "alias")