MAR-189: Refactor hostname pattern for backup_client role (buster/bullseye/bookworm):

- Multiple releases one after the other have names that begin with
letter b, making it necessary to use more than just one letter.
- Use full distro codename, but shorten the parameters to param in
order to avoid issues related to 32-character username limits (when
bak-HOSTNAME user gets created).
- Drop some leftover references to Debian Stretch.
- Drop the arch from hostnames - default is to use 64-bit boxes
anyway.