Description of problem:
dnf system-upgrade doesn't upgrade if packages are downloaded with "--downloaddir" 

Steps to Reproduce:
1. dnf system-upgrade --releasever 27 --downloaddir /mnt/disk1/sysup --downloadonly
2. dnf system-upgrade reboot

Actual results:
Upgrade starts but after a few seconds the system reboots and starts in Fedora 26

Expected results:
System completes upgrade and starts in Fedora 27

Additional info:
dnf system-upgrade log doesn't show any logs and I can't find other logs.
I modified the systemd startup script to remove "reboot" after the process complete and I saw an error about connecting to mirror site.
I proceeded by adding "-C" to the script and I saw a bunch of errors about checksum failures cause system-upgrade is searching for packages in the default location and not in the directory provided on the command line.
On the same system I was able to upgrade using default download directory, not using --downloaddir and --downloadonly options