Delete select-mirror and all the args

It turns out that we pass a lot of args around simply for the
purpose of doing mirror selection. This goes away if everything
gets a mirror.

This change should only be merged after the parent change has
propagated to all puppeted slaves and updated nodepool images.

Co-Authored-By: Jeremy Stanley <fungi@yuggoth.org>
Change-Id: I4a721f6559d7160c8213268a8c1e35c667a67a4f