Add --registry-append and --registry-replace qualifier to docker daemon

These options setup a search list of registries from which to pull images.

Registries will be searched in reverse order.

--registry-append will allow you to add additional registries to docker.io to
pull images from.

For example

--registry-append="foo.com/registry,bar.com/registry"

Would cause a
docker pull myapp

to search
bar.com/registry/myapp
foo.com/registry/myapp
docker.io.../myapp

This would allow companies and vendors to specify registries which contain
content that the companies will not allowed to be stored at docker.io

--registry-replace="foo.com/registry,bar.com/registry"
Would do the same, but not search the docker.io registry.

This would allow a customer to control which sites that images can be pulled from.

Docker-DCO-1.1-Signed-off-by: Daniel Walsh <dwalsh@redhat.com> (github: rhatdan)