Add make `tidy-docker` to clean up build-tools (#192)
Add make `tidy-docker` to clean up build-tools (#192)

* Add make `tidy-docker` to clean up build-tools

This cleans up build-tools images, minus the one in use. Please be
aware that build iamges on your system prior to the labeling of
the build-tools container will require manual removal.

* Correct label prefix to correctly used labels

istio/common-files,istio/common-files