dockerbuilder should delete the image from the host once the push is done

[smothiki]

reduces storage on the host

[arschles]

I'm reopening this for beta4, since it was reverted in #53 (in response to #52).

The issue with the change in #39 was that client.remove_image(image=registry+'/'+imageName, force=True, noprune=False) was removing the image from the registry, not the local docker daemon.

I think, but am not sure (and haven't tested) that client.remove_image(image=imageName, force=True, noprune=False) will delete the image from the local docker daemon.