Ensure availability command output.

Add output for ensure-availability command.

The ensure-availability command will notify the user, how many state servers
were added to the environment.

$ juju ensure-availability -n 7
maintaining machines: "0"
adding machines: "1", "2", "3", "4", "5", "6"

This is a move to github of the MP started in https://codereview.appspot.com/99670044/
Do you mean the missing serialization name for yaml? Or would you prefer more explicit serialization names? If so, would machines-added, etc. suit you?