GCR support for different regions

[devdattakulkarni]

Currently CloudARK uses 'us.gcr.io' as the endpoint for storing container images. We should support customization of endpoint.

[devdattakulkarni]

A related question is that when we do cld gcloud setup, we allow any gcloud region to be set. How does the location of registry affects the region that the app runs in? Does it matter? We should investigate this.