Default to letsencrypt because the upstream default may change.
Passing --staging is no longer needed, since --serever will
select a staging server if needed.
Signed-off-by: Georgi Valkov <gvalkov@abv.bg>
Tested-by: Georgi Valkov <gvalkov@abv.bg>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>