Change 'Uploading context' wording

This changes the wording in the ``docker build`` output from "Uploading context" to the less ambiguous "Sending build context to docker daemon". As discussed in #2342, "Uploading" has negative connotations when the daemon is running locally and what is being sent isn't altogether clear from output alone. Fixes #2342

Docker-DCO-1.1-Signed-off-by: Ryan Stelly <ryan.stelly@live.com> (github: FLGMwt)