registry: Do not push to mirrors
registry: Do not push to mirrors

This patch splits LookupEndpoints into LookupPullEndpoints and
LookupPushEndpoints so that mirrors added with --registry-mirror are
skipped in the list returned by LookupPushEndpoints.

Fixes https://github.com/docker/distribution/issues/823

Signed-off-by: Tibor Vass <3f1326dff4d85ad4ec938fb33f64575ace96db26@docker.com>

thaJeztah/distribution,thaJeztah/distribution