Ubuntu image tests failures

Hey all! We've had this test suite in our internal pipelines for a while now when building/testing Ubuntu cloud images for GCE. However, there are a few tests that repeatedly fail, and on further inspection I believe they're either false positives or "not applicable" to Ubuntu. What we're doing internally at the moment is skipping these tests (using the `--exclude` flag on `docker run gcr.io/gcp-guest/cloud-image-tests...`) but the issue with this is that it skips the whole suite, which is a shame as there are good tests in these suites that are also being skipped. I've added details below of the failing tests - please let me know if you need any more info. All the best! Chlo * `verifySSHConfig()` fails due to `PermitRootLogin` apparently not being set (`image_security_test.go:133: "PermitRootLogin" was not set to "no" or "without-password"`). However, in Ubuntu [the default is `prohibit-password`](https://manpages.ubuntu.com/manpages/focal/en/man5/sshd_config.5.html)? * `TestHostsFile()` fails due to `/etc/hosts does not contain host record` but I think this test should be skipped for Ubuntu? FWIW here's an example GCE Ubuntu `/etc/hosts` file: ```` ubuntu@kajiya-testing:~$ cat /etc/hosts 127.0.0.1 localhost # The following lines are desirable for IPv6 capable hosts ::1 ip6-localhost ip6-loopback fe00::0 ip6-localnet ff00::0 ip6-mcastprefix ff02::1 ip6-allnodes ff02::2 ip6-allrouters ff02::3 ip6-allhosts 169.254.169.254 metadata.google.internal metadata ```` * `TestArePackagesLegal()` surfaces a couple of errors: `/usr/share/doc/libcanberra0/copyright`, `/usr/share/doc/libnspr4/copyright` and `/usr/share/doc/python3-serial/copyright` - we may need to increase the scope of the `var licenses` strings globs? * `TestGetentPasswdOsloginUser()` and `TestGetentPasswdOsloginUID()` both surface: `getent command failed exit status 2` while `TestGetentPasswdAllUsers()` returns: `getent passwd output does not contain sa_105020877179577573373:*:3651018652:3651018652::/home/sa_105020877179577573373` - I'll be honest I'm not sure why this one is failing at first glace, but FWIW we do test `getent` internally on GCE images :smile: (We verify `"getent passwd {} | cut -d: -f6".format(user)` for each user in the instance's metadata ssh keys). * `testShutdownScriptTimeLinux()` returns `shut down time is 89 which is less than 110 seconds` - I understand the sentiment for this test but we also test this internally as well. We run an instance with a `shutdown-script` and check that the correct file was created. However, our internal cutoff time is `30 seconds` instead.