[Docs] Update the screenshot on Zabbix (role and password)

[Irina Povolotskaya]

In 6.1, Zabbix is removed from the core.

This means, you should update:
- https://docs.mirantis.com/openstack/fuel/fuel-6.1/user-guide.html#assign-a-role-or-roles-to-each-node-server - zabbix shouldn’t be on the list, operating system is not present in the list
- https://docs.mirantis.com/openstack/fuel/fuel-6.1/user-guide.html#resetting-the-zabbix-password - same here about Zabbix usage