Make ensure-availability a bulk API call.

Makin ensure-availability a bulk API call.
I'd just do:

```go
args := params.StateServersSpecs{
    Specs: []params.StateServersSpec{{
        EnvironTag:           c.st.EnvironTag(),
        NumStateServers: numStateServers,
        Constraints:           cons,
        Series:                   series,
}}}
```