Comment 2 for bug 1778391

Thanks for filing the bug.
Here's my best guess:

Cloud-init's ds-identify isn't installed[1], which prevented cloud-init from detecting it was running on OpenStack. It eventually got there but it appeared that networking wasn't fully up by the time that it crawled the OpenStack URLs, and eventually the last datasource EC2 ran, and by that time, networking was up and the metadata URLs responded (OpenStack metadata service is compat with EC2 urls) so Ec2 Datasource was used but the system UUID didn't indicate that it was an Ec2 instance (because it isnt) so the warning was displayed.

We've recently fixed an issue with cloud-init upstream's packaging on redhat/fedora/centos where the ds-identify tool wasn't installed where it was expected.

https://bugs.launchpad.net/cloud-init/+bug/1833264

So it may be that's needed to get things working.

1. /run/cloud-init/ds-identify.log
/usr/lib/systemd/system-generators/cloud-init-generator normal=/run/systemd/generator early=/run/systemd/generator.early late=/run/systemd/generator.late
kernel command line (/proc/cmdline): BOOT_IMAGE=/ostree/fedora-atomic-8c75464537441e8641e67a04f0057750eb832264563daf35e6a720c854bcd5e6/vmlinuz-4.16.14-300.fc28.x86_64 no_timer_check console=tty1 console=ttyS0,115200n8 console=ttyAMA0 console=hvc0 net.ifnames=0 rd.lvm.lv=atomicos/root root=/dev/mapper/atomicos-root ostree=/ostree/boot.0/fedora-atomic/8c75464537441e8641e67a04f0057750eb832264563daf35e6a720c854bcd5e6/0
kernel_cmdline found unset
etc_file found unset
default found enabled
checking for datasource
no ds-identify in /usr/lib/cloud-init/ds-identify. _RET=found
enabled via /run/systemd/generator.early/multi-user.target.wants/cloud-init.target -> /lib/systemd/system/cloud-init.target