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/
fwiw, you can make this much more compact:

    for i, test := range []struct{
        // fields
    } {{
        // first test
    }, {
        // second test
    }, {
        // ...
    }} {
        c.Logf("test %d: %s", i, test.about)
        // ...
    }

and while I'm not dogmatic about compressing it *all* into a single structure I would definitely recommend making the struct anonymous and just jamming it into a `var tests = []struct{` construction.