Start shifting balance of bare nodes to trusty

We are beginning to migrate projects to bare-trusty as their default
test slave nodes. As part of this change shift the balance of bare nodes
from precise to trusty ending up with a minimum of 21 nodes for all bare
nodes.

Change-Id: I0f8113e2d333fe42845555518c0ac9a29ee6fe23
Merge "Start shifting balance of bare nodes to trusty"