subtests.docker_cli.start: Fix the conversion to subSubTest

During the conversion of this test the `self` instead of `self.parent_subtest`
was passed to the framework functions causing failures.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
dockertest.containers: Make --size optional

If/when callers or subclasses depend on this data,
they may change the instance attribute ``get_size`` to ``True``.
When ``False``, the size field simply contains a ``None`` value.

To not break existing interface, the default value is ``True``
but will likely change in the near future.

Signed-off-by: Chris Evich <cevich@redhat.com>
dockertest.subtest: Make config_section optional

*  For all existing subtests, this instance attribute
   can easily be derrived from the relative filesystem location
   of the test.  Though this is not always the case, when
   possible, attempt to auto-generate the string.  When that
   fails, pull in the defaults.

Signed-off-by: Chris Evich <cevich@redhat.com>