libcontainer: add method to get cgroup config from cgroup Manager

`configs.Cgroup` contains the configuration used to create cgroups. This
configuration must be saved to disk, since it's required to restore the
cgroup manager that was used to create the cgroups.
Add method to get cgroup configuration from cgroup Manager to allow API users
save it to disk and restore a cgroup manager later.

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
+}

libcontainer/container_linux_test.go

@@ -61,6 +61,9 @@ func (m *mockCgroupManager) GetUnifiedPath() (string, error) {
 func (m *mockCgroupManager) Freeze(state configs.FreezerState) error {
 	return nil
 }
+func (m *mockCgroupManager) GetCgroups() (*configs.Cgroup, error) {
+	return nil, nil
+}
 
 func (m *mockIntelRdtManager) Apply(pid int) error {
 	return nil
@@ -82,6 +85,10 @@ func (m *mockIntelRdtManager) Set(container *configs.Config) error {
 	return nil
 }
 
+func (m *mockIntelRdtManager) GetCgroups() (*configs.Cgroup, error) {
+	return nil, nil
+}
+
 type mockProcess struct {
 	_pid    int
 	started uint64