internal/resource: Support gcs urls in Ignition config

[sohankunkerkar]

Fixes #707
I could see the following information on the serial console log after performing some testing around these changes.

[    7.930150] ignition[678]: DEBUG    : parsing config with SHA512: 05f47789410ba7da511ce86c0dc350ba4a7f736092530044097f8611656ffc297b351eed2b5141e7736ef2bd8ba992a95c6f8c0a55a906142cf464265c12beb1
[    7.932253] ignition[678]: DEBUG    : Warning: falling back to unauthenticated GCS access: google: could not find default credentials. See https://developers.google.com/accounts/docs/application-default-credentials for more information.
[    7.934666] ignition[678]: DEBUG    : fetched referenced config at gs://coreos-ignition-test/test.ign with SHA512: cf83e1357eefb8bdf1542850d66d8007d620e4050b5715dc83f4a921d36ce9ce47d0d13c5d85f2b0ff8318d2877eec2f63b931bd47417a81a538327af927da3e

[coreosbot]

Can one of the admins verify this patch?

[bgilbert]

Have you tested authenticated fetch of a non-public object?

[sohankunkerkar]

Have you tested authenticated fetch of a non-public object?

Yes, just confirmed it's working fine for the older code. Let me test it again after incorporating your suggestions.

[sohankunkerkar]

Have you tested authenticated fetch of a non-public object?

Yes, just confirmed it's working fine for the older code. Let me test it again after incorporating your suggestions.

This looks good. No deviations from the previous finding.

[jlebon]

Note that the OpenCensus version we vendored in this patch is causing issues in early boot systems with low entropy: census-instrumentation/opencensus-go#1228.

If we get that (or something functionally equivalent) merged there, then let's update the vendoring here to get that in.

[arithx]

Great work tracking that down @jlebon

[jlebon]

If we get that (or something functionally equivalent) merged there, then let's update the vendoring here to get that in.

This is done in #1107.