Ensure passwordless ssh with stack between nodes

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
1 file changed