Allow secondary to restore from primary while primary archives to swift

[Loïc Gomez]

We need to be able to have the secondary restore from primary, while having the primary archive to swift storage.

This means we would need restore_command to stay unset, even if postgresql.wal_e.enabled is true.
I'd suggest adding a new boolean option, wal_e_restore_on_standby or similar around https://git.launchpad.net/postgresql-charm/tree/reactive/postgresql/replication.py#n564

Thank you,
Loïc