MAR-151: Added support for Debian 10 Buster to xmpp_server role:

- Updated role reference documentaiton.
- Updated role meta information.
- Updated tests.
- Enable lower-level TLS protocols (1.0/1.1) in global OpenSSL
configuration file on Buster in order to be able to test the
xmpp_server_tls_protocol parameter (otherwise Prosody completely
refuses to use them even if listed in its configuration).
- Move stretch-specific tests into its own file (for backported
lua-ldap library), and run them on Debian 9 Stretch machines only.