<check-sources> Add package exclusion handling

Added `exclude` field to `RepoTuple`

Added logic to parse excludes from ini files/default `RepoDB` dataset

Added `merge_excludes` to `check_sources.py`, sets repo `exclude`
option to existing value(s) + any new values provided

Added `set_excludes` to `oo-admin-check-sources.py` and call from
`run_checks`. Repos that are required and available and have `exclude`
set in the `RepoDB` object will have their package exclusions modified
accordingly.