(WIP) (PUP-2150) Fix yumrepo backwards compatibility issues.

Previously the yumrepo type properly supported setting properties
to absent and having them be appropriately deleted from the files.
This work readds this functionality by extending inifile to allow
us to delete entries, as well as always mark the file dirty if it
didn't exist already, making sure even just a blank [x] file can
be written out.