Configure cloud-init to avoid fighting with puppet changes

Every time virtual machines generated with cloud-init (e.g. the staging hosts) restart, cloud-init will clobber some changes made to them by puppet.

We should configure cloud init to preserve the changes made by puppet, by disabling its intervention on the following:

apt sources
/etc/hosts (?)
/etc/network/interfaces (?)

The relevant config is in /etc/cloud/cloud.cfg (there seems to be a drop-in directory /etc/cloud/cloud.cfg.d that we could use in each profile that touches a "conflicting" file)

Migrated from T2655 (view on Phabricator)