Hello,
 
You were able to install on stretch?

I was attempting to do so, but the python3-django-auth-ldap package wasn't available on anything but sid.

In order to install on Stretch, you should activate the stretc backports as explained here https://validation.linaro.org/static/docs/v2/installing_on_debian.html#installing-on-debian-stretch

So:
1/ adding "deb http://deb.debian.org/debian stretch-backports main" to apt source
2/ apt update
3/ apt -t stretch-backports install lava-server


Regards

-- 
Rémi Duraffort
LAVA Team
Linaro