Separating cgroup Memory and MemoryReservation.

This will allow for these to be set independently. Keep the current Docker behavior where Memory and MemoryReservation are set to the value of Memory.

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)
Merge pull request #5373 from vmarmol/master

Separating cgroup Memory and MemoryReservation.