MAR-151: Switch to Debian 10 Buster in usage instructions:

- Specify that Debian Buster should be used instead of Debian Stretch.
- Switch to using https links where possible.
- Minor fixes to wording/instructions.
- Updated link towards Debian Buster preseed documentation appendix.
- Use Python 3 when serving the preseed files.
- Fixed DNS subject alternative name for the XMPP server (it should be
the domain served by the XMPP server, not its canonical FQDN).
- Added small note for database_server role and how it sets-up root
account authentication, and update instructions for logging-in into
database server as root.