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.
Perhaps we could these slightly more descriptive, so that we know what package they've come from at any rate. e.g.

var (
    ErrConflict = errgo.New("elastic search document conflict")
    ErrNotFound = errgo.New("elastic search document not found")
)

?