I am using mcollective-2.8.1 version and installed all the related packages, but its seems like shell command is missing. I am not able to find the package/plugin for shell command.
MCollective Shell is a separate Agent plugin, it is not packaged with the core MCollective software. Agent plugin code is installed into $libdir/mcollective/agent. See the general plugin install guide. You can get the Agent code from GitHub.