planet: pre-load gcr.io containers for k8s

[klizhentas]

Right now planet pulls gcr.io containers (e.g. pause) from the internet, pre-load containers in planet before start, so that planet can operate without internet:

2m        2s        18        mattermost-database   Pod       implicitly required container POD   Failed    {kubelet 172.28.128.5}   Failed to pull image "gcr.io/google_containers/pause:0.8.0": i

How success looks like

• Turn off internet and create a new site using gravity and planet. The planet should start up

[kontsevoy]

Additional data: newer versions of Docker complain about gcr.io Docker registry saying that it's old and soon to be disabled.

[a-palchikov]

Yes. In the long run, we can always build our own images.

[a-palchikov]

Merged changes with #74