ncm-grub: do not return an error but a warning if kernel version is a magic value

Valid magic values are empty string or 'YUM-managed' (backward compatibility with the template library).

Minimalistic change to avoid ncm-grub triggering a rerun of ncm-ncd at every new
profile received. Note that in this case, ncm-grub does essentially nothing.

This fix is mainly intended to make https://github.com/quattor/ncm-cdispd/pull/6 usable before a major rewrite of `ncm-grub` (#232). This is an alternative to #231 to suplly a minimalistic fix to #158. Choose one or the other!