[#59323898] Move CDN logs to separate mount

The existing path belongs to the root partition, which isn't very big. Move
it to `/srv/logs/log-1` which is created by `ci_environment::transition_logs`
and lives on a separate partition:
```
dcarley@transition-logs-1:~$ df -h / /srv/logs/log-1/
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda2       9.4G  2.7G  6.3G  31% /
/dev/sdb1       252G   54G  199G  22% /srv/logs/log-1
```

The old dir/log should be deleted after deploying.
Merge pull request #104 from alphagov/59323898-cdn_logs_on_separate_mount

[#59323898] Move CDN logs to separate mount