[mod] add Ubuntu 20.10 image to the LXC suite

Ubuntu 20.10 image [1] is avialable from https://images.linuxcontainers.org

BTW: replace builhost package 'ttf-dejavu' by more common meta package
'fonts-dejavu' ('ttf-dejavu' is no longer available in 20.10).

[1] https://github.com/lxc/lxc-ci/commit/d161de601bffe4055d77b9d52a826ce5d81567d9



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>