Provide ability to just run existing image template

Currently the action takes either the local JSON or the action parameters and creates and uploads an image template and then runs it. We'd like to manage our image template definitions along with other ARM template deployments and only use the action to initiated the run. Using the image-builder-template-name parameter it would be good to check if there is a template which already exists by that name and use it rather than attempting to re-submit the same template.

Effectively we'd need a test [here](https://github.com/Azure/build-vm-image/blob/2f65ddeb473a967617f896253017aaef024d1730/src/ImageBuilder.ts#L116) to see if the image template already exists before trying to put the image template