scripts/k8s-access: allow to disable access methods

When using the bootstrap tag we use the cluster-proxy/ssh for all zones and not only the zone that is currently deployed. Which is not really what we want.

Ex: a config require access to its parent cluster. In this case we can use rancher2.

Instead of using the bootstrap tag to determine how to access the cluster we check the availability of all methods in a specific order.

First rancher2, then if available cluster-proxy and finally ssh tunnel.

By default all method are checked, but some can be disabled if the config requires it.