Update plugin load and snapshot service

Allow plugins to be mapped and returned by their ID.
Add skip plugin to allow plugins to decide whether they should
be loaded.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

server/config.go

@@ -17,11 +17,6 @@ type Config struct {
 	Debug Debug `toml:"debug"`
 	// Metrics and monitoring settings
 	Metrics MetricsConfig `toml:"metrics"`
-	// Snapshotter specifies which snapshot driver to use
-	Snapshotter string `toml:"snapshotter"`
-	// Differ specifies which differ to use. Differ is tightly coupled with the snapshotter
-	// so not all combinations may work.
-	Differ string `toml:"differ"`
 	// Plugins provides plugin specific configuration for the initialization of a plugin
 	Plugins map[string]toml.Primitive `toml:"plugins"`
 	// Enable containerd as a subreaper