Persistent directory for container in execdriver

This is needed for persistent namespaces

Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com>

daemon/execdriver/driver.go

@@ -51,6 +51,7 @@ type Driver interface {
 	Info(id string) Info                          // "temporary" hack (until we move state from core to plugins)
 	GetPidsForContainer(id string) ([]int, error) // Returns a list of pids for the given container.
 	Terminate(c *Command) error                   // kill it with fire
+	Clean(id string) error                        // clean all traces of container exec
 }
 
 // Network settings of the container