The Avocado command line application has "subcommands". So far,
to execute these commands, a special method, named "dispatch"
was set on the command line parser result object (aka Namespace)
pointing to the `run` method of the same class.

In truthfullness, the command line parser doesn't have too much
to do with these methods. Also, the `set_defaults` method of the
command line argument parser is not exactly intended for that use.
So, let's ask the subcommands subparser to formally save the chosen
subcommand, at well, `subcommand`, and nothing more.

Since the application knows about the command plugins (the ones that
inherit from CLICmdBase), and their interface now *require* a method
called `run`, let's use the extension entry-point name to locate the
underlying object and its `run` method.
Signed-off-by: NCleber Rosa <crosa@redhat.com>