datasource/gce: add initial support

[crawford]

No description provided.

[crawford]

$ sudo ./coreos-cloudinit -oem=gce
2016/05/18 20:13:48 Checking availability of "gce-metadata-service"
2016/05/18 20:13:48 Fetching user-data from datasource of type "gce-metadata-service"
2016/05/18 20:13:48 Fetching data from http://metadata.google.internal/computeMetadata/v1/instance/attributes/user-data. Attempt #1
2016/05/18 20:13:48 Fetching meta-data from datasource of type "gce-metadata-service"
2016/05/18 20:13:48 Fetching data from http://metadata.google.internal/computeMetadata/v1/instance/network-interfaces/0/access-configs/0/external-ip. Attempt #1
2016/05/18 20:13:48 Fetching data from http://metadata.google.internal/computeMetadata/v1/instance/network-interfaces/0/ip. Attempt #1
2016/05/18 20:13:48 Fetching data from http://metadata.google.internal/computeMetadata/v1/instance/hostname. Attempt #1
2016/05/18 20:13:48 Merging cloud-config from meta-data and user-data
2016/05/18 20:13:48 Set hostname to crawford-ignition-test-1.c.coreos-gce-testing.internal
2016/05/18 20:13:48 Writing file to "/etc/environment"
2016/05/18 20:13:48 Wrote file to "/etc/environment"
2016/05/18 20:13:48 Updated /etc/environment
2016/05/18 20:13:48 Ensuring runtime unit file "etcd.service" is unmasked
2016/05/18 20:13:48 Ensuring runtime unit file "etcd2.service" is unmasked
2016/05/18 20:13:48 Ensuring runtime unit file "fleet.service" is unmasked
2016/05/18 20:13:48 Ensuring runtime unit file "locksmithd.service" is unmasked

$ cat /etc/environment 
COREOS_PRIVATE_IPV4=10.240.0.14
COREOS_PUBLIC_IPV4=104.196.121.100

[marineam]

What happens if the machine has no external IP?

[marineam]

Hint: it shouldn't fail ;-)

[crawford]

Now it doesn't!

[marineam]

LGTM