Trouble stacking mdadm, multipath and lvm at boot

[E V <eliventer@gmail.com>]

I'm trying to get mdadm to assemble an array on some multipath disks
at boot. System is a fresh install of stretch. By default it seems the
mdadm array auto assembles before the multipath devices are created
and thus doesn't use the dm- multi path devices but just the sdx
devices, which prevents the multipath devices from ever getting
created. Adding the DEVICE lines in mdadm.conf for the
/dev/mapper/mpathx devices kind of reverses the problem. Now the
multipath devices do get created, but the array(and thus the lvol &
filesystem) never gets assembled on boot. In the old init days I'd
probably add a script that runs mdadm --assemble and mount at the end
of boot, but not sure what the right way to handle this in systemd is.
mdadm.conf lines:

DEVICE /dev/mapper/mpathb /dev/mapper/mpathc /dev/mapper/mpathd
/dev/mapper/mpathe
ARRAY /dev/md/0
devices=/dev/mapper/mpathb,/dev/mapper/mpathc,/dev/mapper/mpathd,/dev/mapper/mpathe
metadata=1.2 spares=1 name=tst3:0
UUID=7809f500:667e7a8f:45222d03:7405415b


Thoughts or suggestions?

Thanks,
-Eli