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)

cgroups/cgroups.go

@@ -19,6 +19,8 @@ type Cgroup struct {
 	Memory       int64 `json:"memory,omitempty"`        // Memory limit (in bytes)
 	MemorySwap   int64 `json:"memory_swap,omitempty"`   // Total memory usage (memory + swap); set `-1' to disable swap
 	CpuShares    int64 `json:"cpu_shares,omitempty"`    // CPU shares (relative weight vs. other containers)
+
+	UnitProperties [][2]string `json:"unit_properties,omitempty"` // systemd unit properties
 }
 
 type ActiveCgroup interface {
@@ -87,5 +89,14 @@ func writeFile(dir, file, data string) error {
 }
 
 func (c *Cgroup) Apply(pid int) (ActiveCgroup, error) {
-	return rawApply(c, pid)
+	// We have two implementation of cgroups support, one is based on
+	// systemd and the dbus api, and one is based on raw cgroup fs operations
+	// following the pre-single-writer model docs at:
+	// http://www.freedesktop.org/wiki/Software/systemd/PaxControlGroups/
+
+	if useSystemd() {
+		return systemdApply(c, pid)
+	} else {
+		return rawApply(c, pid)
+	}
 }