Add convenience symlink to integration test output

[holmanb]

Context:

Cloud-init allows automatic log collection from integration tests, however the UX isn't very friendly if tests are being ran repeatedly. Selecting "which directory" is not fun. Consider:

holmanb@arc:~ > ls /tmp/cloud_init_test_logs -l
total 36
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 10:52 211110105102
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 11:32 211110113108
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 11:38 211110113715
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:37 211110143557
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:39 211110143852
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:41 211110144125
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:47 211110144700
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:48 211110144823
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 15:22 211110152216

How to choose? (obviously a mix of tab completion and ls -1 output makes it not-horrible, but it could certainly be better)

The most common use case is "look at the last run". This change makes that convenient.

holmanb@arc:~ > ls /tmp/cloud_init_test_logs -l
total 36
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 10:52 211110105102
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 11:32 211110113108
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 11:38 211110113715
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:37 211110143557
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:39 211110143852
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:41 211110144125
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:47 211110144700
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:48 211110144823
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 15:22 211110152216
lrwxrwxrwx 1 holmanb holmanb   38 Nov 10 15:22 last -> /tmp/cloud_init_test_logs/211110152216

holmanb@arc:~ > ls /tmp/cloud_init_test_logs -l  
total 40
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 10:52 211110105102
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 11:32 211110113108
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 11:38 211110113715
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:37 211110143557
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:39 211110143852
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:41 211110144125
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:47 211110144700
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 14:48 211110144823
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 15:22 211110152216
drwxrwxr-x 3 holmanb holmanb 4096 Nov 10 15:23 211110152312
lrwxrwxrwx 1 holmanb holmanb   38 Nov 10 15:23 last -> /tmp/cloud_init_test_logs/211110152312

Now there is no choice to make for the common use case. /tmp/cloud_init_test_logs/last/ is the answer.

Commit message:

Add convenience symlink to integration test output

Integration test runs get unique log directories at
/tmp/cloud_init_test_logs/$DATE_TIME. Make
/tmp/cloud_init_test_logs/last always point to the most recent
integration test directory.

Test:

Run any integration test, watch /tmp/cloud_init_test_logs/last/ appear as a symlink to latest test.
Run the test again, watch the symlink update.

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