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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>