Avoiding unintended MDRaid activation on server install

Hello everyone,

I'm facing one persistent issue that I've been trying to solve for quite 
some time now:

If the clients' disks have been incorrectly/insufficiently wiped, a 
subsequent install might fail after creating the initial disk layout, as 
at that moment, leftover mdraid/lvm metadata get autodetected by either 
the kernel or the rd (not sure which is responsible for this), and 
activated, instanciating them as md/lvm devices.

If the setup-storage routine then attempts to create a filesystem on 
their underlying block devices, it fails with a "Device or resource busy"

Example layout:

disk_config sda disklabel:gpt-bios
primary -   1024    -   -
primary -   10240   -   -
primary -   0-      -   -

disk_config sdb sameas:sda

disk_config raid
raid1   swap    sda1,sdb1   swap    sw
raid1   /       sda2,sdb2   ext4    noatime,errors=remount-ro 
mdcreateopts="--metadata=0.90"
raid1   -       sda3,sdb3   -       -

disk_config lvm
vg disk_tmp    md2
disk_tmp-var          /var                      20G     ext4 
noatime,errors=remount-ro
disk_tmp-home         /home                     50G     ext4 
noatime,errors=remount-ro

disk_config tmpfs
tmpfs   /tmp    RAM:10% defaults


After a server gets installed with this config, if I wipe its disks only 
by running a wipefs -af /dev/sda /dev/sdb, raid signatures are left at 
the same offsets, and will get automatically activated after re-applying 
the same disk partition layout, preventing further device manipulation.

Reading dracut.cmdline manual page, I tried appending rd.md=0 rd.dm=0 
rd.lvm=0 to kernel's cmdline, but to no avail.

Full fai log can be found at: https://pastebin.com/7LNgZ2av

Note: The three errors following the partition.DEFAULT hook are related 
to my feeble attempt at solving this issue by activating all found 
mdraids and wiping all /dev/sd* /dev/vd* and /dev/md* devices 
pre-partitioning. But as the disk doesn't have its initial partition 
layout yet, no md devices are found, and so can't be wiped.

Thank you for any advice you'll be able to provide!

--L. Pavljuk