Want /proc to be mounted when running Ansible

[basil]

@pzakha writes that as an alternative to #582 we could mount /proc when processing the Ansible scripts:

For reference you can see what you are doing in set_bootfs_not_mounted() in upgrade/upgrade-scripts/rootfs-container. The Ansible scripts are launched from live-build/config/hooks/configuration/80-build-configuration.binary, so perhaps we could just do:

mount --rbind /proc binary/proc
mount --make-rslave binary/proc

ansible-playbook -vv -c chroot -i binary, ansible/playbook.yml

umount -R binary/proc

If this issue is fixed, then #582 can be closed. If #582 is fixed, then this issue can be closed.