For compute migration to work, the stack user needs to be configured
with passwordless ssh between all hosts involved in the migration.
Reuse the build ssh-key for this, which is already distributed for
user root.
Depends-on: https://review.openstack.org/563584
Change-Id: Id07f55fea06509466add35315c135dbfba6aa714
(cherry picked from commit afe1414019)