cluster_run should output an appropriate connection string for the cluster

There has been a few cases of engineers being confused by what ports they need to use when connecting to a cluster_run-started cluster with our SDKs and other tools.  Ideally, cluster_run should print out an appropriate connection string for use with all tools and SDKs.  A specification for connection string can be found here: https://github.com/couchbaselabs/sdk-rfcs/blob/master/rfc/0011-connection-string.md.

Essentially it should print out something like this:
`couchbase://127.0.0.1:12000,127.0.0.1:12001,127.0.0.1:12002`