fix(coreos-base/oem-gce): block_until_url and coreos-setup-environment

Curl will not retry after most errors, just connection timeouts and some
HTTP errors. In our case the most likely error is network unreachable.
Instead use our simple-minded but trusty block_until_url script to wait.

The setup-environment script should be named coreos-setup-environment so
that it is called by the standard coreos-setup-environment.service