centos: Update CentOS documentation
centos: Update CentOS documentation

SLES documentation was created based on the CentOS document. Some
updates were made to add more clarity to the installation steps. This
commit adds the updates back to the CentOS guide.

Fixes: #864

Signed-off-by: Erick Cardona <7e10327bafb63098d26367a1914879bbea067098@intel.com>

clearcontainers/runtime,devimc/runtime,chavafg/runtime,devimc/runtime,chavafg/runtime,clearcontainers/runtime