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>

services/snapshot/default_linux.go

@@ -0,0 +1,5 @@
+package snapshot
+
+const (
+	defaultSnapshotter = "overlayfs"
+)