Support modification of serverless templates

[mikljohansson]

In order to be able to orchestrate Serverless Runpod.io deployments as part of a continuous deployment workflow it would be desirable to be able to update the Serverless template using runpodctl. Specifically to change the Container image setting on the template to point to a new version of the image.

Pointing the template to the :latest label runs the risk of docker pull caches being out of sync and running an old version of the image. And it makes rollback difficult too.

Ideally I'd like it to be possible to execute a runpodctl command and point it to an existing Serverless template to a new image URL