Move assignment policy to global env config
Previoysly, each provider defined it's own AssignmentPolicy,
which was hard coded to AssignNew. Now, it's been made an
env setting with default AssignNew. With containers, it will be
possible to add a unit to an existing machine (if not dirty) so
this work is a step in that direction.
R=jameinel, fwereade
CC=
https://codereview.appspot.com/9824043