Plugins: add optional plugins directory

The goal of the `optional_plugins` directory is to hold all plugins
that ship with Avocado, but are not considered core functionality.
Plugins that are considered to be core still live under
`avocado/plugins`.

This also adds all directories to be found under `optional_plugins` to
the list of candidate plugins when running `make clean` or `make
link`.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

Makefile

@@ -17,6 +17,7 @@ VERSION=$(shell $(PYTHON) setup.py --version 2>/dev/null)
 DESTDIR=/
 AVOCADO_DIRNAME=$(shell echo $${PWD\#\#*/})
 AVOCADO_PLUGINS=$(filter-out ../$(AVOCADO_DIRNAME), $(shell find ../ -maxdepth 1 -mindepth 1 -type d))
+AVOCADO_PLUGINS+=$(shell find ./optional_plugins -maxdepth 1 -mindepth 1 -type d)
 RELEASE_COMMIT=$(shell git log --pretty=format:'%H' -n 1 $(VERSION))
 RELEASE_SHORT_COMMIT=$(shell git log --pretty=format:'%h' -n 1 $(VERSION))
 COMMIT=$(shell git log --pretty=format:'%H' -n 1)

optional_plugins/README.rst

+==================
+ Optional Plugins
+==================
+
+This is where plugins shipped with Avocado, but not considered core
+functionality can be found.
+
+To try them out on a development environment, you may run::
+
+ $ cd <plugin-dir>/
+ $ python setup.py develop --user
+
+And to remove them on a development environment, you may run, at the
+same directory::
+
+ $ python setup.py develop --uninstall --user
+
+Also, on a development environment, the following command on the
+topmost Avocado source code directory will enable all optional
+plugins::
+
+ $ make link
+
+And this will disable all optional plugins::
+
+ $ make unlink