Remove the image from the agent machine after pushing

[varshavaradarajan]

The docker registry artifact plugin is invoked after building an image normally. Or pulling down an image to push elsewhere. Once the image is pushed, there is no need for the image to be present on the agent.

Provide an option to remove the docker image from the host. This can be disabled if elastic agents are being used (exception: elastic agents used with Docker outside of Docker) and there are no chances of the image being present on the host machine.