cc_seed_random: unexpectedly appends cloud's random_seed to specified seed data

[Dan Watkins]

The test_seed_random_data test specifies this user-data:

#cloud-config
random_seed:
  data: 'MYUb34023nD:LFDK10913jk;dfnk:Df'
  encoding: raw
  file: /root/seed

and asserts that that is the exact content of /root/seed. If we're using a datasource which sets random_seed in its metadata (i.e. Azure and OpenStack, at least), this test fails: cc_seed_random will _append_ the cloud's random_seed to the given data before writing it out.

To be clear, the bug here is that this is unexpected: we do not document this behaviour. I can't think of a compelling case for not behaving in this way: more entropy is good, and we have write_files if users really do want to write out exact content to a path.

The TestSeedRandomData.test_seed_random_data integration test assumes the "exact writing" behaviour, so will also need to be updated.

[Dan Watkins (oddbloke)]

I've just opened a PR for this: https://github.com/canonical/cloud-init/pull/771

[Dan Watkins (oddbloke)]

The PR has landed, as this is a doc and test change, we don't need to wait for a release to mark this Fix Released.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3827