Do not install Ansible PPA on Ubuntu 20.04

Do not install Ansible PPA on Ubuntu 20.04

Unfortunately the Ansible PPA is not yet available on Ubuntu 20.04 so
do not install it. Also add a TODO note to go back and figure out if
we really still need the PPA. This was added because we wanted to get
a more recent version of Ansible on Ubuntu 16.04 but perhaps the distro
packaged version is sufficient with newer Ubuntu versions.

Ref: https://github.com/ansible/ansible/pull/69161
Signed-off-by: Thanh Ha <zxiiro@gmail.com>
Change-Id: I7a02a6e0e95b148c640dfd41f3ea3d50dfff17d9