Fixes for Ubuntu 12.04 (#15) and CentOS 6.5 (#12, #14) images.

build_scripts/run_centos.sh: Update OS version, cosmetic modifications to image description
build_scripts/run_ubuntu.sh: idem
centos/centos-6-minimal-ks-1.0.cfg: Keep epel repo enabled, work around python 2.6 email module bug by modifying cloudinit file
ubuntu/ubuntu12_stratus.preseed: Remove /usr/bin/stratuslab-one-context from /etc/rc.local