Fixes #7947 - Add option to validate package checksums when syncing

Added an option to the config to check the package checksums after they are
synced. This will tell if packages are corrupt, partially downloaded, etc. We
decided to make it a config option as it decreased the performance of the sync
by about 40% (observed 1 hr 12 min without validation for a RHEL 6server
kickstart repo without validation and 1 hr 40 min for the same repo but with
the checksum validation enabled).