let cloud-init manage /etc/hosts on container

this avoids the message 'unable to resolve host: uoi-bootstrap' from sudo.
this fixes #294 

Note that the lxc prep script for the ubuntu-cloud template apparently sets this by default, but doesn't set it if you use your own userdata:
https://github.com/lxc/lxc/blob/f50b163d1d565a9c5f3fbab725b999c5746961ad/hooks/ubuntu-cloud-prep#L141