fix invalid memory limit error issue

Sandbox was reusing LXC config files, which had more generous memory
limits (because of compiling) than the problem allowed for. Also, it was
allowing an extra 10MB of memory without telling outside. Therefore,
some submissions used the full amount of memory available in the VM and
later punished as memory limit exceeded errors.

To remedy this, we generate LXC config every time, and explicitly
specify memory limit every time we call Sandbox.run()

See https://algospot.com/forum/read/2494/