avocado.cli.app: Fix to display all --help options.

Fix the way that we handle argument parsing, to display
all options for subcommands when --help is passed.

We fix the problem by using parents parse.

Plus: cosmetic fixes in avocado.plugins regarding
argument parse.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #146 from ruda/fix_help_option

avocado.cli.app: Fix to display all --help options.