Missing an option to push to an insecure registry.

It's common for development and integration tests

Log:

time="2018-05-14T18:59:03Z" level=info msg="pushing layers" exporting the image 10.200.10.1:5000/tsuru/go:latest failed: failed to do request: Head https://10.200.10.1:5000/v2/tsuru/go/blobs/sha256:a440082623eba20362e20fea387de304e5fe1d6cce355e7189e39ed7d79fdc07: http: server gave HTTP response to HTTPS client