MAR-132: Added support for Debian 9 (Stretch) to database_server role:

- Updated Molecule test configuration to include Debian 9 Stretch in
test matrix.
- Updated tests related to UTF-8 configuration (differences between
Debian 8 and 9).
- Deploy UTF-8 configuration in alternate locations depending on what
distro is being used.
- Force set-up of root password on Debian Stretch (default is to use
the unix_socket authentication).