Add daemon flag to specify public registry mirrors

Adds support for a --registry-mirror=scheme://host[:port]
daemon flag. The flag may be present multiple times. If
provided, mirrors are prepended to the list of endpoints used
for image pull. Note that only mirrors of the public
index.docker.io registry are supported, and image/tag resolution
is still performed via the official index.

Docker-DCO-1.1-Signed-off-by: Tim Smith <timbot@google.com> (github: timbot)
As this is an array, what happens when you have more than one specified - I'm presuming that we ask each in turn 'do you have it', but which one downloads it if neither have it?