Pushing documentation for new CentOS installation method.

The new documentation resolves issues GH-575 and GH-586:

1. The Vagrant installation has been verified to work with VirtualBox 4.3.8, so the warning about not using VirtualBox 4.3.0 or higher has been removed.

2. The Python SCL installation method, which was changed by its maintainers to install Python 2.7.5 for x86_64 architectures only, has been replaced by an altinstall installation procedure that keeps the system on Python 2.7.3 and works for i386 and x86_64. Unlike the SCL installation, the altinstall installation requires the Python installation to be compiled before being installed.

3. An outdated image used in doc/challenge-design-game-admin-smartgrid-game.rst has been replaced with one that shows the current interface for creating a new SGG draft grid.