chore(*): bump CoreOS to 452.0.0

This addresses a security vulnerability with bash.

See the release notes at https://coreos.com/releases/#452.0.0
Revert "docs(contrib/rackspace): add manual update instructions"

This reverts commit f46b7ea17e33f38d32cc8cfa732d010591853db6.

Conflicts:
	contrib/rackspace/README.md
ref(units): replace bash commands with network connections with ncat.
fix(deisctl/units): fix ncat loop in builder and registry
Merge pull request #1940 from carmstrong/coreos_452.0.0

chore(*): bump CoreOS to 452.0.0