Add SELinux mount option to NewMounter() and MountDevice()

Let volume plugins decide if they want to mount volumes with "-o
context=XYZ" or let the container runtime relabel the volume on container
startup.

Using NewMounter, as it's the call where a volume plugin gets the other MountOptions.

pkg/volume/volume.go

@@ -129,6 +129,7 @@ type MounterArgs struct {
 	FsGroup             *int64
 	FSGroupChangePolicy *v1.PodFSGroupChangePolicy
 	DesiredSize         *resource.Quantity
+	SELinuxLabel        string
 }
 
 // Mounter interface provides methods to set up/mount the volume.
@@ -262,7 +263,8 @@ type Attacher interface {
 
 // DeviceMounterArgs provides auxiliary, optional arguments to DeviceMounter.
 type DeviceMounterArgs struct {
-	FsGroup *int64
+	FsGroup      *int64
+	SELinuxLabel string
 }
 
 // DeviceMounter can mount a block volume to a global path.