At this point, no plugin support exists in Avocado. This is in
preparation for the new plugin code to be cleanly introduced.

The plugins that play a role in the plugin architecture have
been removed, since they wouldn't be functional under the new
plugin management code. The ones that are add extra functionality
to Avocado have been kept, and will be ported to the new
architecture.

Also, most of the functional tests have been temporarily disabled.
The reason is that most of them run avocado, which depends on the
run command, which in turn, depends on the plugin archicture code.
Signed-off-by: NCleber Rosa <crosa@redhat.com>