The enable-<cloud> flags should not be set until external provider support is added

In order to ensure that the transition from the in-tree provider to the external provider for each cloud is seamless, the k8s-master charm should hold off on setting the enable-<cloud> flag for each cloud until it has support for that cloud's external provider. This way, the support in cdk-addons can be done and backported when available, and it will continue to work with older charms which don't support the external provider, since those charms will not set the relevant enable flag.