chore(contrib/*): update to CoreOS 324.2.0

Double-check the IDs:
EC2: https://coreos.com/docs/running-coreos/cloud-providers/ec2/
Rackspace: https://coreos.com/docs/running-coreos/cloud-providers/rackspace/
Revert "docs(contrib/rackspace): document manual update for CoreOS"

This reverts commit 20ecc297e8e9e62a4b3ef9c48d82955123258c12.