This PR begins to migrate Conduit to Linkerd2:
* The proxy has been completely removed from this repo, and is now located at
github.com/linkerd/linkerd2-proxy.
* A `Dockerfile-proxy` has been added to fetch the most-recently published proxy
binary from build.l5d.io.
* Proxy-specific protobuf bindings have been moved to
github.com/linkerd/linkerd2-proxy-api.
* All docker images now use the gcr.io/linkerd-io registry.
* `inject` now uses `LINKERD2_PROXY_` environment variables
* Go paths have been updated to reflect the new (future) repo location.
Kevin Lingerfelt
Thomas Rampelberg
Oliver Gould