I want to install packages all over servers at the same time. Finally I will install a cloud service and distributed filesystem for testing purposes?
I found Webmin has Cluster option, but no clue or clear documentation about how to do. Webmin UI is useful for me, I decided to use it whenever I need inventory of any of servers.
All the modules in the Cluster category make use of the Webmin Servers Index module and RPC to control other systems.
Webmin has great cluster documentation. There is cluster option at navigation menu, just start from looking around and reading manual. It will be clearer for you then.