refactor: install check-mk-agent from RHEL package repository instead of the Check_MK server

tasks/install_checkmk_plugins.yml

 ---
 - name: CMK installieren
   yum:
-    name: "https://cmk.slub-dresden.de/slubmon/check_mk/agents/check-mk-agent-2.0.0p12-1.noarch.rpm"
+    name: "check-mk-agent"
     state: present
   when: ansible_os_family == "RedHat"