SSH not allowed if on different organisation

[sandipndev]

Although OS Login (with 2fa) is enabled on the VM instances, if someone outside of the organisation on which the Galoy Stack is being run is given access to the VM, they are not able to SSH even after uploading the SSH keys to gcloud.

To mitigate this, we need to add roles/compute.osLoginExternalUser on bootstrap and grant external SSH access to specific users. Also, this is only valid for projects under an organisation.

Refer:

1. https://cloud.google.com/compute/docs/oslogin/manage-oslogin-in-an-org#external-user
2. https://cloud.google.com/compute/docs/access/iam#compute.osLoginExternalUser