cgroups: Add systemd implementation of cgroups

This implements cgroup.Apply() using the systemd apis.
We create a transient unit called "docker-$id.scope" that contains
the container processes. We also have a way to set unit specific
properties, currently only defining the Slice to put the
scope in.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)

runtime/execdriver/native/default_template.go

@@ -5,6 +5,7 @@ import (
 	"github.com/dotcloud/docker/pkg/cgroups"
 	"github.com/dotcloud/docker/pkg/libcontainer"
 	"github.com/dotcloud/docker/runtime/execdriver"
+	"github.com/dotcloud/docker/utils"
 	"os"
 )
 
@@ -59,6 +60,19 @@ func createContainer(c *execdriver.Command) *libcontainer.Container {
 		container.Cgroups.MemorySwap = c.Resources.MemorySwap
 	}
 
+	if opts, ok := c.Config["unit"]; ok {
+		props := [][2]string{}
+		for _, opt := range opts {
+			key, value, err := utils.ParseKeyValueOpt(opt)
+			if err == nil {
+				props = append(props, [2]string{key, value})
+			} else {
+				props = append(props, [2]string{opt, ""})
+			}
+		}
+		container.Cgroups.UnitProperties = props
+	}
+
 	// check to see if we are running in ramdisk to disable pivot root
 	container.NoPivotRoot = os.Getenv("DOCKER_RAMDISK") != ""