Make ensure-availability a bulk API call.

Makin ensure-availability a bulk API call.
This needs to be:

```go
results := params.StateServerChangeResults{Results: make([]params.StateServersChangeResult, len(args.Specs)}
```
for consistency with other similar API calls.
Then, you can simplify the loop a bit:

```go
var err error
for i, spec := range args.Specs {
    results.Results[i].Result, err = c.ensureAvailabilitySingle(spec)
    results.Results[i].Error = common.ServerError(err)
}

return results, nil
```

Also, very important - the return values of all server-side API calls need to return an error as well as a result. So please reintroduce the error return, even if you always return nil at the end.