Add GPUs as an explicit resource.

We will add "gpus" as an explicitly recognized resource in Mesos, akin to cpus, memory, ports, and disk. In the containerizer, we will verify that the number of GPU resources passed in via the --resources flag matches the list of GPUs passed in via the --nvidia_gpus flag. In the future we will add autodiscovery so this matching is unnecessary. However, we will always have to pass "gpus" as a resource to make any GPU available on the system (unlike for cpus and memory, where the default is probed).