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!
The schema allows the path to be undefined but `ncm-grub` registers for changes to `/system/kernel/version` which is incompatible with having it undefined! The magic value was probably a bad choice, an empty string was probably better. Unfortunately it has been used as a trick until know in the template library and I'd preferred to first release a version of `ncm-grub` that allows both, then update the template library to use the empty string, then remove the magic value support in a future major rewrite of `ncm-grub` (that I agree to take in charge if nobody else wants to do it). I'd like to avoid synchronizing multiple changes if possible thus this staging approach.