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>

cmd/containerd/config_linux.go

@@ -15,7 +15,5 @@ func defaultConfig() *server.Config {
 			Level:   "info",
 			Address: "/run/containerd/debug.sock",
 		},
-		Snapshotter: "io.containerd.snapshotter.v1.overlayfs",
-		Differ:      "io.containerd.differ.v1.base-diff",
 	}
 }

cmd/containerd/config_unix.go

@@ -14,7 +14,5 @@ func defaultConfig() *server.Config {
 			Level:   "info",
 			Address: "/run/containerd/debug.sock",
 		},
-		Snapshotter: "io.containerd.snapshotter.v1.naive",
-		Differ:      "io.containerd.differ.v1.base-diff",
 	}
 }

cmd/containerd/config_windows.go

@@ -17,7 +17,5 @@ func defaultConfig() *server.Config {
 			Level:   "info",
 			Address: `\\.\pipe\containerd-debug`,
 		},
-		Snapshotter: "io.containerd.snapshotter.v1.windows",
-		Differ:      "io.containerd.differ.v1.base-diff",
 	}
 }