Changes

distribution: modify warning logic when pulling v2 schema1 manifests
The warning on pull was incorrectly asking to contact registry admins. 
It is kept on push however.
Pulling manifest lists with v2 schema1 manifests will not be supported
thus there is a warning for those, but wording changed to suggest
repository author to upgrade.
Finally, a milder warning on regular pull is kept ONLY for DockerHub
users in order to incite moving away from schema1.
Signed-off-by: Tibor Vass <tibor@docker.com>