Curthooks should run system_upgrade before apply_net to allow upgrading cloud-init before detecting passthrough support

[Wesley Wiedenmeier]

Curtin detects whether or not apply_net can use passthrough networking based on the version and features of cloud-init in-target. It may be desirable to run curthooks.system_upgrade() before calling apply_net(). This allows us to test how curtin interacts with development versions of cloud-init and allows users to upgrade the in-target version of cloud-init via a ppa or custom repo to one that supports passthrough networking features that they want.

The change to curthooks should be minor, and should not affect other behavior.

[Ryan Harper (raharper)]

Indeed.

I also noticed that we don't have a stage where we might run in-target
hooks prior to calling apply_net so even with a PPA there isn't a way
to upgrade the target with a newer cloud-init.

[Wesley Wiedenmeier (wesley-wiedenmeier)]

Even without the in-target hooks I have been able to get a run in using cloud-init from my ppa by adding it with the apt_config. The config entry I'm using is:
system_upgrade:
    enabled: true
apt:
    preserve_sources_list: false
    sources:
        cloud-init-test-repo.list:
            source: "deb http://ppa.launchpad.net/wesley-wiedenmeier/test/ubuntu xenial main"
            keyid: 2B91B36D

This works to get cloud-init upgraded in target before starting the tests, but in-target hooks before apply-net would give us a lot more flexibility on other modifications we might want to make.