CentOS support: basic support for centos 6.4

Centos 6.4 basic support. Samples included in ./config/vm-centos.*

It consists of few modification of ./build/init, taking care of
using yum+rpm in place of apt-get+deboostrap. Code for managing
grub 0.99 came with that commit.

The image may be generated using make make pxe DIST=centos DVER=6.4
in the build directory. However you should use D7 initrd image for
deployment. Sample includes post-conf snippets for networking.

Only the base role is working, support for additionnal role will
come later.
Merge branch 'master' of https://github.com/enovance/edeploy