Ignore hotplug socket when collecting logs (SC-345)

[TheRealFalcon]

Proposed Commit Message

Ignore hotplug socket when collecting logs

Update "cloud-init collect-logs" to ignore
/run/cloud-init/hook-hotplug-cmd as this will raise the error
"/run/cloud-init/hook-hotplug-cmd` is a named pipe" if included.

Also updated logs.py to continue writing the tarball if it fails
collecting a file rather than let the exception bubble up.

LP: #1940235

Additional Context

https://bugs.launchpad.net/cloud-init/+bug/1940235

Test Steps

Start an impish image (or packages/bddeb, upload to your instance, install deb and reboot).
Run cloud-init collect-logs
On main, fails as mentioned in the bug. On this branch, shows Wrote /root/cloud-init.tar.gz having hook-hotplug-cmd excluded from the tarball. From now on, if any other error is encountered, we should see something along the lines of:

Failed collecting file(s) due to error:
[('/run/cloud-init/hook-hotplug-cmd', '/tmp/tmpbsl70bx1/cloud-init-logs-2021-08-17/run/cloud-init/hook-hotplug-cmd', '`/run/cloud-init/hook-hotplug-cmd` is a named pipe')]
Wrote /root/cloud-init.tar.gz

with the error file(s) being excluded from the tarball.

We should really add integration tests for cloud-init collect-logs. I'd like to add it to this PR if there's time.

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[paride]

Not an authoritative review but this lgtm!

[blackboxsw]

This works for me. We'll get a noise error if other files are root read-only in /run/cloud-init (which only cloud-init should be writing anyway).