libcontainer: add method to get cgroup config from cgroup Manager

Add method to get cgroup configuration from cgroup Manager to allow other
packages and aplications save it in disk.

fixes #2176

Signed-off-by: Julio Montes <julio.montes@intel.com>

libcontainer/cgroups/cgroups.go

@@ -46,6 +46,9 @@ type Manager interface {
 
 	// Sets the cgroup as configured.
 	Set(container *configs.Config) error
+
+	// Gets the cgroup as configured.
+	GetCgroups() (*configs.Cgroup, error)
 }
 
 type NotFoundError struct {

libcontainer/cgroups/fs/apply_raw.go

@@ -467,3 +467,7 @@ func CheckCpushares(path string, c uint64) error {
 
 	return nil
 }
+
+func (m *Manager) GetCgroups() (*configs.Cgroup, error) {
+	return m.Cgroups, nil
+}

libcontainer/cgroups/systemd/apply_nosystemd.go

@@ -61,3 +61,7 @@ func (m *Manager) Freeze(state configs.FreezerState) error {
 func Freeze(c *configs.Cgroup, state configs.FreezerState) error {
 	return fmt.Errorf("Systemd not supported")
 }
+
+func (m *Manager) GetCgroups() (*configs.Cgroup, error) {
+	return nil, fmt.Errorf("Systemd not supported")
+}

libcontainer/cgroups/systemd/apply_systemd.go

@@ -528,3 +528,7 @@ func isUnitExists(err error) bool {
 	}
 	return false
 }
+
+func (m *LegacyManager) GetCgroups() (*configs.Cgroup, error) {
+	return m.Cgroups, nil
+}

libcontainer/cgroups/systemd/unified_hierarchy.go

@@ -346,3 +346,7 @@ func (m *UnifiedManager) Set(container *configs.Config) error {
 	}
 	return nil
 }
+
+func (m *UnifiedManager) GetCgroups() (*configs.Cgroup, error) {
+	return m.Cgroups, nil
+}