Adapt central logserver config for EL-7.

Rsyslog version 8, which is used in RHEL-7, requires some changes
to the rsyslog.conf file compared to what we could do in version 5
(the version used in RHEL-6):

 * BSD-style blocks of rules are not supported.  These are blocks
   starting with a line starting with a "+", "-" or "!", and we
   used "+*" and "+<%= @hostname %>" for rules applying to logs
   from anywhere, or from the local host.
   Instead, we need to use RainerScript ``if'' statements checking
   the value of $fromhost-ip.

 * A single asterisk ("*") to mean send a message to all logged-in
   users no longer works.  Instead we need to explicitly select the
   omusrmsg output module, and write it as ":omusrmsg:*".

 * Use the imjournal input module, and don't use the imklog input
   module.

 * Explicitly set $WorkDirectory.