Harmonize the use of add_main_repository

Prior to this commit Debian installation were using add_main_repository,
but RHEL platforms were still using echo "REPOURL". The echo command
has been moved to the function add_main_repository so all platforms
use the same pattern
Move to CentOS 6.5

Install CentOS 6.5 instead of CentOS 6.4
Merge remote-tracking branch 'spredzy/el_platform_add_main_repo'