Explicit region configuration in skylark init

[parasj]

Region management:

• skylark init should show the list of enabled regions to the user and ask them to confirm. This will fix the issue where listing GCP instances queries regions where a user does not have either Compute Engine or GCS enabled.
• It will write those regions to ~/.skylark/aws_regions, etc. The user can edit these lists if needed.
• If a cloud command fails in a specific region, do not catch the exception; we should surface this to the user.

Explicitly disable clouds:

• If the user disables a cloud provider, skylark init should set a flag in the config to denote that the cloud is disabled. The cloud-specific auth classes should read this file to determine if a cloud is enabled.
• The cloud auth classes should have no "magic"; if a cloud is disabled, do not attempt to auto-load credentials (e.g. google.auth.default)
• This is different from what Fix Issue #211 #213 is doing; we should return errors to the user without catching them. Remove the try-catch from skylark/compute/gcp/gcp_auth.py.