Ignore .log files

Those files are created when one tries to build an image and a stemcell:

```shell
$ git status
On branch master
Your branch is up to date with 'origin/master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)

    ci/docker/os-image-stemcell-builder/ubuntu-bionic-18.04-cloudimg-console.log
```

.gitignore

@@ -11,6 +11,8 @@ stemcell_builder/public_stemcell_config.yml
 src/github.com/cloudfoundry/stemcell-acceptance-tests/stemcell
 src/github.com/cloudfoundry/stemcell-acceptance-tests/syslog-release
 
+**/*.log
+
 tags
 cscope.out