Remove cloud_init role

[sjpb]

The creation of a new cloud_init role to enable creating /etc/hosts at boot time was a misfeature;

• The workflow was nasty: It required provisioning ports and creating an inventory with terraform, then running ansible to template cloudinit userdata, then running terraform again to provision servers, then running ansible for site.yml
• The mechanism for adding to userdata (e.g. to add block device/volume configuration) was nasty/non-obvious.

Something similar may still be required for non-environment-specific images but that requirement has gone away right now, so this PR removes the cloud_init role entirely and uses a bootstrap.yml step to template /etc/hosts if required. Note that the packer-built environment-specific compute images are still usable for slurm-controlled reboots without DNS, as they contain the templated /etc/hosts.