Allow devicePlugin.config.default to be none.

[nikp1172]

This commit allows someone not to set a clusterwide policy for GPUs It will take the default behaviour. Only if a node has selector for a particular devicePlugin config (with a given name). This config will be used

Hello!

Thanks for making this contribution! When contributing to this repository please keep in mind the following:

• You should sign your work.
• You should be making your contribution against the gitlab.com repository as github.com is just a mirror.

[shivamerla]

@nikp1172 can you submit the PR here instead: https://gitlab.com/nvidia/kubernetes/gpu-operator