Append group permissions instead of clearing them

We set the groups for the syslog user to be "adm", but this will clear
any existing groups. That will mess with permissions in cases where we
want syslog to be in other groups for the ability to handle a different
applications logging.

Change-Id: If267cfd2f3f8f182c396e3f2836ec7e1d572fe0a

tasks/rsyslog_client_pre_install.yml

@@ -23,6 +23,7 @@
     name: "syslog"
     groups: "adm"
     system: yes
+    append: yes
     shell: /bin/false
     state: present