Code review comment for ~oddbloke/cloud-init/+git/cloud-init:apt-pipelining

> On which releases can we set this to 'os' ?

We no longer run the broken mirrors for any release, and the apt pipelining fix has landed in xenial and later. (Debian stable has a more recent version of apt than xenial, so I infer it's also landed there.)

> Further, does apt automatically do this or does one have to opt-in via 'os'?

The previous default of False would cause us to write out a configuration file which would definitely disable pipelining. Using one of "os", "none" or "unchanged" as the value will cause us to _not_ write out that configuration file, giving us the default behaviour configured in the underlying OS image.