Better messaging when local cluster is already running

[mfburnett]

If you run spread cluster start and the cluster had already been started, there's no message that tells you so. It would be helpful to have a message to also remind people to switch back to the correct kubectl context for localkube. I'm thinking something like:

Localkube is already running.  

You may need to switch your kubectl context if you switched to using a different cluster:
$ kubectl config use-context localkube