$ sudo vi /etc/apt/sources.list  (add a line : deb http://http.debian.net/debian stretch-backports main)
$ sudo apt update
$ sudo apt -t stretch-backports install lava-server
$ sudo a2dissite 000-default
$ sudo a2enmod proxy
$ sudo a2enmod proxy_http
$ sudo a2ensite lava-server.conf
$ sudo service apache2 restart 

$ sudo service lava-server restart
$ sudo service lava-coordinator restart
$ sudo service lava-master restart
$ sudo service lava-server-gunicorn restart
$ sudo service apache2 restart

----
My versions:
$ dpkg -l lava-server

Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                                          Version                     Architecture                Description
+++-=============================================-===========================-===========================-===============================================================================================
ii  lava-server                                   2017.7-1~bpo9+1             all                         Linaro Automated Validation Architecture server


$ dpkg -l lava-dispatcher
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                                          Version                     Architecture                Description
+++-=============================================-===========================-===========================-===============================================================================================
ii  lava-dispatcher                               2017.7-1~bpo9+1             amd64                       Linaro Automated Validation Architecture dispatcher


----
Then
visist http://127.0.0.1:8000
and meet errors "Internal Server Error" and found errors in lava related logs.


