Use select-mirror in devstack-gate

The select-mirror logic has grown more complex, but devstack-gate's
mirror selection has not kept up. Instead of duplicating, just reuse the
existing script.

This change depends on:

  https://review.openstack.org/59196
  https://review.openstack.org/58153

Change-Id: Icbc240b83961834083f6044b30abb7f1d7321306
Merge "Use select-mirror in devstack-gate"