(maint) 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 and @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.