Ansible

Ansible is a configuration management system written in Python. It can be used for automating machine deployments.

Emerge
Install :

Configuration
The app-admin/ansible-1.9.4 ebuild will not generate the basic configuration. The default inventory file is named hosts should be in the /etc/ansible directory

This file contains the managed computers organized in groups.

With "ansible_ssh_user=" and "ansible_ssh_port=" remote users and ssh ports can be specified per hosts

Example usage
You can use ansible-doc to read module documentation

Roles for Gentoo
You can find over 40 roles specifically for Gentoo on https://github.com/gentoo-ansible.