Add output for ensure-availability command.

state/api/params/params.go

 // Copyright 2013 Canonical Ltd.
 // Licensed under the AGPLv3, see LICENCE file for details.
 
 package params
 
 import (
         "bytes"
         "encoding/json"
         "fmt"
 
@@ skipping 713 matching lines @@
 
 // EnsureAvailability contains arguments for
 // the EnsureAvailability client API call.
 type EnsureAvailability struct {
         NumStateServers int
         Constraints     constraints.Value
         // Series is the series to associate with new state server machines.
         // If this is empty, then the environment's default series is used.
         Series string
 }
+
+// EnsureAvailabilityResult lists the servers
+// that have been added, removed or maintained in the
+// pool as a result of an ensure-availability operation.
+type EnsureAvailabilityResult struct {

[fwereade, 2014/06/02 09:50:03]

Surely, this is not an anythingResult? It's a StateServerChange, or something,
isn't it?

While you're at it, would you change the name of EnsureAvailability please?
That's a verb, not a noun; I'm drawing a bit of a blank on a better name at the
moment, but things like StateServerRequest and StateServersSpec are floating
around my head.

Regardless, I'd expect us to be accepting/returning arrays of same; and the
structures in play to look something like:

type StateServerChange struct { ... }
type StateServerChangeResult struct {
    Result *StateServerChangeResult
    Error  *Error
}
type StateServerChangeResults struct {
    Results []StateServerChangeResult
}

+        Added      []string
+        Maintained []string
+        Removed    []string
+}