subtests.docker_cli: Add new test load_similar

Fixes: https://github.com/autotest/autotest-docker/issues/346

This test creates image with file /testfile. Than it saves this image
and modifies the content of the /testfile. Then it loads the original
image and verifies the file contain correct value. Without any changes
it loads the corrupted version and verifies that the value was modified.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Maybe it's too tricky but this could be nice:

```python
self.stuff['img_id'] = self.stuff.get('img_id',
                                      self.cmd_output("cat %s/img_id" % self.srcdir))
```

Just an idea.