avocado.plugins: Plugins has priority.

Add priority when loading plugins (both built-in or external).
The lowest the priority (0 by default) then earliest the plugin will
be loaded.

In the current implementation, only the runner plugin has
priority 0 and the other plug-ins are all defined as priority 3.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
avocado.plugins.runner:  Export the test runner parser.

Export the test runner parser back to the main parser,
so you can use `parser.runner` to extend it.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
avocado.plugins: Move plugins to the runner.

Move plugins journal, json, xunit and vm inside the test runner
subcommand, now the command line options belongs to the runner,
instead of the main application.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
seltests: Fix tests regarding moving options to the runner.

Fix functional tests regarding the changes that moves
options from the main application to the test runner (avocado run).

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