Add support for Sandbox API gateway

[tobiaslohr]

This adds support for working with On-Demand Sandboxes against the new Sandbox API Gateway. Benefits of this include:

• Avoids messing around with cluster configuration - Knowledge of the cluster your sandbox realm is provisioned on is not needed and abstracted
• Managing sandboxes provisioned on multiple clusters

Example:

You have sandboxes on realms in multiple clusters. Prior to this change you have to set the env var SANDBOX_API_HOST to the respective host name of the sandbox cluster before managing sandboxes, e.g. sfcc-ci sandbox:list. After that to see the sandboxes in your other realm provisioned in the other cluster, you set the env var to a different value and call sfcc-ci sandbox:list again.

Now, with this change you don't have to set the env var at all, but just call sfcc-ci sandbox:list and you are able to manage the sandboxes of both realms from both clusters.

[salesforce-cla]

Thanks for the contribution! It looks like @tobiaslohr is an internal user so signing the CLA is not required. However, we need to confirm this.