With docker, users have an option to specify memory size when
creating containers. This patch bring this capability to Magnum.
Specifying memory size is optional but recommended. If memory size
is unset, magnum will set mem_limit as None on docker create.
Change-Id: Ife88284402ff39a7b2e051f6e66913e502f2ba8c
Closes-Bug: #1487224