<check-sources> Bug 1028612, fix excludes list building logic, RHN code

https://bugzilla.redhat.com/show_bug.cgi?id=1028612

List `need_exclude` in `set_excludes` wasn't being built properly, it
failed to filter by repos which already had excludes set. Added
`_excludes_needed` helper function to build this list.

`set_save_repo_attr` in `OpenShiftCheckSources` didn't handle writing
configs from iterables for RHN correctly. Added simple logic to munge
`value` variable if it's iterable

Also included several spelling, formatting and minor refactors from
code review with Miciah.