subtests.docker_cli: Add ps_size test

Issue https://github.com/autotest/autotest-docker/issues/254

Tested on Fedora:
```
14:06:29 INFO | 0.0 0 1.0
14:06:29 INFO | 1.049 1 1.1
14:06:29 INFO | 10.49 10 2.0
14:06:29 INFO | 104.9 100 11.0
14:06:29 INFO | 1099.776 1000 101.0
14:06:29 INFO | 5497.856 5000 501.0
```

and with busybox:
```
14:33:11 INFO | 0.0 0 1.0
14:33:11 INFO | 1.049 1 1.1
14:33:11 INFO | 10.49 10 2.0
14:33:11 INFO | 104.9 100 11.0
14:33:11 INFO | 1099.776 1000 101.0
```

This test executes couple of containers, creates file in each of them
and compares the size of the container with the size of the created file.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Could you toss these into a configuration seciton in ``index.rst`` for the subtest?