Now that we have ubuntu-bionic images online, we can start gating for
python3.6.
We only need to install bubblewrap PPA for ubuntu-xenial,
ubuntu-bionic already has bubblewrap packaged.
Update bindep.txt skipping ubuntu-xenial for bubblewrap.
Change-Id: Ic850a07001a9adef9cc459fcb107e0ebaaddf8ab
Depends-On: https://review.openstack.org/565884
Depends-On: https://review.openstack.org/565948
Signed-off-by: Paul Belanger <pabelanger@redhat.com>