(#9293) Yumrepos should be ensurable

This change adds a provider to yumrepo that makes it ensurable.
So far we needed todo a workaround making each of the individual files
that yumrepo creates ensurable, now yumrepo itself does it. 

This probably should belong in 3.x as well as in 2.7x

https://projects.puppetlabs.com/issues/9293