dockertest.containers: ValueError on invalid names

In case we need to introduce variations in container
name parsing, moved code into separate method.  Added
some simple checks that raise ValueError on obviously
badly formatted names.  Note however that the class
*does* still permit a "None" name, so this case is
allowed through.  Added unittests to cover positive
and negative name values.

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