feat(coreos-base/oem-gce): support cloudconfig

With this pull request we can support a cloudconfig sent to the
user-data metadata key. Tested etcd and fleet bootstrapping and ssh key
insertion.
fix(coreos-base/oem-*): be consistent about sh -e

Use sh -e everywhere to be safe.
fix(coreos-base/oem-gce): use only one into

Suggestion by @marineam.
Merge pull request #472 from philips/more-gce-oem-setup

feat(coreos-base/oem-gce): support cloudconfig