…rator and network-configurator

This is to make them fail under the same circumstances, e.g. do not generate
systemd units corresponding to network interfaces that we cannot write any
network configuration files for, due to some late-stage validation error.

Adding new (internal, for now)  _netplan_state_get/set_flags() API.

This is enabled by passing a new NETPLAN_STATE_VALIDATION_ONLY flag to
NetplanState, to skip writing out files.
- The sd-generator (./generate) should not write any network configuration, but still needs to run through its late-stage validations, to fail if the network-configuration would fail.
- The network-configuration should not write any systemd units, but still needs to run trhough its late-stage validations, in order to fail if the sd-generator would fail.

Future improvement:
Ideally, there should not be any late-stage validation inside file writing logic (e.g. [gen-]networkd.c, [gen-]openvswitch.c, ...), but all validation should already be done in the central validation.c stage. Unfortunately, such change is out of scope for this PR.

…r integration during 'netplan try'

…#2090848)

This avoids conflicts with AppArmor confinement when re-generating the network
configuration from within NetworkManager.

See original PR: canonical#162

…on of the test

It introduces unrelated failures and wastes resources, as we're not currently
affected by an ABI breaking change.

…nerator

This was originally implemented to generate & start systemd units just-in-time
during the boot transaction (canonical#162).

With the implementation of a proper systemd-generator, Netplan generates the
corresponding units in /run/system/generator* and automatically re-loads and
re-calculates dependencies during "daemon-reload". Therefore, we do not need
to inject them manually.

This is covered by the "cloud-init" autopkgtest.

sbuild does not provide a fully functional systemd environment, as indicated
by the existance of /run/systemd/system. Therefore, we cannot rely on the
systemd-run sandbox in such environments.