hooks: change the semantic of NVIDIA_VISIBLE_DEVICES=""
With LXC, you can override the value of an environment variable to
null, but you can't unset an existing variable.
The NVIDIA hook was previously activated when NVIDIA_VISIBLE_DEVICES
was set to null. As a result, it was not possible to disable the hook
by overriding the environment variable in the configuration.
The hook can now be disabled by setting NVIDIA_VISIBLE_DEVICES to
null or to the new special value "void".
Signed-off-by: Felix Abecassis <fabecassis@nvidia.com>