avocado.cli.parser: New way to parsing arguments.

Modify how we parse command line arguments,
by removing the logic that was inside avocado.cli.app into
this new module.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
avocado.plugins: Use avocado.cli.parser and method run().

On Plugin.configure(), remove app_parser and cmd_parser arguments
and now use parse, an instance of avocado.cli.parser.Parser().
The application (old app_parser) is available with parser.application.
The subcommands (old cmd_parser) is available with parser.subcommands.

Now the runner plugins (like sysinfo, datadir, run, etc.)
make use of the standard method run() to execute the logic.

Signed-off-by: Ruda Moura <rmoura@redhat.com>