fix: nocloud no fail when network-config absent

[blackboxsw]

Proposed Commit Message

Commit 5322dca2 introduced an assumption to read_seeded that
network-config must always be present for NoCloud datasource.
Since it is still considered and optional supplemental configuration
allow the read_seeed calls to succeed in the absence of network-config.

Avoids failures seen in tests/integration-tests/datasources/test_nocloud.py::
  test_nocloud_seedfrom_vendordata

Additional Context

Test Steps

Failing integration test without this changeset

CLOUD_INIT_KEEP_INSTANCE=1 CLOUD_INIT_CLOUD_INIT_SOURCE=ppa:cloud-init-dev/daily CLOUD_INIT_OS_IMAGE=oracular CLOUD_INIT_PLATFORM=lxd_container tox -e integration-tests -- tests/integration_tests/datasources/test_nocloud.py::test_nocloud_seedfrom_vendordata

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[holmanb]

One nitpick: "Error in network-config response" may make users think that something is wrong, when it is not. "No network-config provided" may make more sense to an end user.

Thanks @blackboxsw!