Add CentOS support (in addition to Ubuntu)

This PR adds support for CentOS in the `guest_script.sh`. It aligns the install procedures in order to keep the variations in the install process as small as possible.

As part of that I switched from the Git based installation process to `pip install ansible`. This was mainly because the Git setup was not working for me out of the box (don't remember the error anymore) and for CentOS we anyway needed to install pip (to install the ansible Python dependencies).

Successfully tested it both with a CentOS 6.5 and Ubuntu 14.04 box.