DownloadLinuxRelease downloads the release linux binary to the given directory.
FindBinary searches for an Envoy debug binary under ISTIO_OUT. If the ISTIO_OUT environment variable is not set, the default location under GOPATH is assumed. If ISTIO_OUT contains multiple debug binaries, the most recent file is used.
FindBinaryOrFail calls FindBinary and fails the given test if an error occurs.