programatically enforce minimum requirements for yum configuration including debug messages

Currently it is possible to enable YUM support (and in fact it is enabled by default in pro) even if no createrepo command is found.

This can lead you down the track of configuring stuff for yum support and filling up the logs with exceptions that no createrepo command was found.

It would be better to simply disable the capability if no command is found and not allow enabling it either, and instead presenting a useful error message in the UI that says you need to have createrepo and mergerepo on the path (and probably therefore run on Linux..)

It should probably look for the mergerepo command as well.