2 or more logging fixes

Make the use of @log_dir consistent …
Previously, /var/log/@name, @log_dir, and @log_dir || /var/log/puppetdb were
all in use. As @log_dir is always defined when the templates get evaluated,
they don't need a default case to fall back to. This commit makes the use of
@log_dir/@name consistent in the templates.

(#20148) Prevent warnings during logrotate using su …
Because the log dir is 775 and has the group of puppetdb (to facilitate
puppetdb writing logs), logrotate warns and skips rotating puppetdb logs. This
commit adds a su directive as suggested in the redmine ticket to calm logrotate
and allow it to successfully rotate puppetdb's logs, in spite of the
permissions.