Comment # 7 on bug 1017695 from Andrei Borzenkov

The problem seems to be this patch

Fri Oct 14 14:20:51 CEST 2016 - hare@suse.de

- 90mdraid: Use stock MD rules to assemble RAID arrays (bsc#998860)
  *add 0313-90mdraid-Use-stock-MD-rules-to-assemble-RAID-arrays.patch

Stock MD rules require
/usr/lib/systemd/system/mdadm-last-resort@.{service,timer} to trigger start of
degraded array. These are missing in initrd.

Workaround is to add /etc/dracut.conf.d/mdadm.conf with content

install_optional_items+=" /usr/lib/systemd/system/mdadm-last-resort@.service
/usr/lib/systemd/system/mdadm-last-resort@.timer "

and rebuild initrd. If system is already stuck, boot with rd.break=pre-mount
and manually run "mdadm --manage --run /dev/mdX" for all mdX (root and resume).
You may need to do it once more after boot for all other arrays after entering
emergency mode.

This is actually upstream bug. Files are missing upstream as well.