Improve handling of OS and kernel version definition with sl5/6.x templa...

...tes.

Kernel version (/system/kernel/version) is now defined by kernel_version_arch
template, except when using sl5/6.x templates to let YUM manage grub.conf.
Due to ncm-grub oddities, in this case the kernel is defined to something
non existent (leading to a ncm-grub error that can be ignored).