adjust get-addons-templates to handle new repo url for nvidia-device-plugin

[Kevin W Monroe]

The nvidia-device-plugin template has changed the image url from docker.io to nvcr.io/nvidia/k8s-device-plugin:

https://github.com/NVIDIA/k8s-device-plugin/commit/9a3d866a96e2b0cc5f252a84734e45b75a82de88

Update our get-addons-templates script to substitute nvcr.io with our custom registry config.

[Kevin W Monroe (kwmonroe)]

PR for review:

https://github.com/charmed-kubernetes/cdk-addons/pull/201

[Cory Johns (johnsca)]

Cherry-picked to release-1.20: https://github.com/charmed-kubernetes/cdk-addons/commit/504d91c300d88e38e7c84e3f23f97cd30d14df81

[Cory Johns (johnsca)]

Cherry-picked to 1.19 (https://github.com/charmed-kubernetes/cdk-addons/commit/5395312341b083092fbb23d76f8600aa58086dd3) and 1.18 (https://github.com/charmed-kubernetes/cdk-addons/commit/55dc13226d40712504ab76a44b51b5b21f13d72b) as well. Thanks for the catch, George.

[Kevin W Monroe (kwmonroe)]

1.20+ck2 is no longer planned; this will go out with the upcoming 1.21 release.