test/e2e_node/system/types_unix: support ZFS

Docker validation tests in the case of ZFS used as the graph driver
fail due to "zfs" not being present in the default Docker specification.

Add "zfs" in the GraphDriver slice.

test/e2e_node/system/types_unix.go

@@ -63,7 +63,7 @@ var DefaultSysSpec = SysSpec{
 	RuntimeSpec: RuntimeSpec{
 		DockerSpec: &DockerSpec{
 			Version:     []string{`1\.1[1-3]\..*`, `17\.03\..*`}, // Requires [1.11, 17.03]
-			GraphDriver: []string{"aufs", "overlay", "overlay2", "devicemapper"},
+			GraphDriver: []string{"aufs", "overlay", "overlay2", "devicemapper", "zfs"},
 		},
 	},
 }