lava-dispatcher-host: support to install on ubuntu focal

Although it is not officially supported, lava-dispatcher-host can be
installed on ubuntu focal and works well.

The installation is broken since the depends `base-files (<< 11)`
introduced for supporting the default cgroups v2 on debian bullseye.

Here are the base-files versions on ubuntu focal, debian buster and
bullseye.

```
vagrant@lava-focal:~$ apt-cache madison base-files
base-files | 11ubuntu5.5 | http://archive.ubuntu.com/ubuntu
focal-updates/main amd64 Packages
base-files |  11ubuntu5 | http://archive.ubuntu.com/ubuntu focal/main
amd64 Packages

vagrant@lava-buster:~$ apt-cache madison base-files
base-files | 10.3+deb10u12 | http://deb.debian.org/debian buster/main
amd64 Packages
base-files | 10.3+deb10u12 | http://deb.debian.org/debian buster/main
Sources

vagrant@lava-bullseye:~$ apt-cache madison base-files
base-files | 11.1+deb11u3 | http://deb.debian.org/debian bullseye/main
amd64 Packages
base-files | 11.1+deb11u3 | http://deb.debian.org/debian bullseye/main
Sources
```

With depends `base-files (<< 11.1)`, the lava-dispatcher-host can be
installed on all these three releases. The same as debian buster, ubuntu
focal uses cgroups v1 by default.

Signed-off-by: Chase Qi <chase.qi@linaro.org>
19 jobs for lava-dispatcher-host-focal in 17 minutes and 8 seconds (queued for 7 seconds)
latest
Status Name Job ID Coverage
  Test
passed dispatcher-debian-10 #153663
amd64

00:03:02

passed dispatcher-debian-11 #153665
amd64

00:03:04

passed server-debian-10 #153664
amd64

00:03:37

passed server-debian-11 #153666
amd64

00:05:43

 
  Analyze
passed bandit-sast #153667
amd64-dind

00:00:52

passed black #153670
amd64

00:00:46

passed code_quality #153671
amd64

00:00:24

passed codespell #153676
amd64

00:00:20

passed coverage #153672
amd64

00:06:59

61.01%
passed dockerfiles #153673
amd64

00:00:17

passed eslint-sast #153668
amd64-dind

00:00:26

passed pylint #153675
amd64

00:02:37

passed schemas #153674
amd64

00:00:31

passed semgrep-sast #153669
amd64-dind

00:00:51

 
  Build
passed debian/10 #153678
amd64

00:01:31

passed debian/11 #153679
amd64

00:01:34

passed doc #153677
amd64

00:00:39

passed docker-amd64-dispatcher #153680
amd64-dind

00:03:50

passed docker-amd64-server #153681
amd64-dind

00:04:20