This adds new style mirror_info handling to use-docker-mirror to give us
greater control over whether or not docker hub should be mirrored. We
ignore old style configuration if new style is present which gives us
this control. Otherwise we fallback to the old behavior.
We also update the ensure-docker test jobs to be triggered by updates to
the use-docker-mirror roles as ensure-docker includes this role. We
should get decent functional testing coverage this way.
Change-Id: Ia1b216a6dd68bcafbe599777037c5d7b1b3e8201
b007e86a0f