Handle memory swappiness as a pointer to handle default/unset case

This prior fix to set "-1" explicitly was lost, and it is simpler to use
the same pointer type from the OCI spec to handle nil pointer == -1 ==
unset case.

Also, as a nearly humorous aside, there was a test for MemorySwappiness
that was actually setting Memory, and it was passing because of this
bug (as it was always setting everyone's MemorySwappiness to zero!)

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

spec.go

@@ -398,7 +398,8 @@ func createCgroupConfig(name string, spec *specs.LinuxSpec) (*configs.Cgroup, er
 			c.Resources.KernelMemory = int64(*r.Memory.Kernel)
 		}
 		if r.Memory.Swappiness != nil {
-			c.Resources.MemorySwappiness = int64(*r.Memory.Swappiness)
+			swappiness := int64(*r.Memory.Swappiness)
+			c.Resources.MemorySwappiness = &swappiness
 		}
 	}
 	if r.CPU != nil {