Automatically initialise elasticsearch.

Charmstore will now automatically initialise elasticsearch when starting up.
This ensures that elasticsearch will always have the correct index and mapping
configuration required for correct operation.
or:

    func mustJSON(data string) interface{} {
        var x interface{}
        if err := json.Unmarshal([]byte(data), &x); err != nil {
            panic(err)
        }
    }
    var esIndex = mustJSON(`
    { .... }`)

    var esMapping = mustJSON(`
    { ....}`)

?

To save a bit of runtime space, we could keep them as *json.RawMessage rather than interface{} too, although there may be other reasons why that won't work.