(PUP-3024) Add deltarpm_percentage and deltarpm_metadata_percentage

This patch adds deltarpm_percentage and deltarpm_metadata_percentage to the yumrepo type.
These yum repo configuration options are only available with newer versions of Fedora and CentOS 7, which have DeltaRPM support (Presto).

Earlier versions of yum appear to ignore options which aren't recognised, loading the repo file successfully.

- Does there need to be added logic which rejects this parameter for earlier OS versions? (This could be quite a bit of work and other modules such as the puppetlabs-apt one are not doing something similar currently)

- Should I also create some kind of (doc) tracking ticket for this as well?