remove shutdown logic from PluginManager and add a add_cleanup() API

for the already existing cleanup logic of the config object.
This simplifies lifecycle management as we don't keep two
layers of shutdown functions and also simplifies the pluginmanager
interface.

also add some docstrings.

--HG--
branch : plugin_no_pytest

testing/test_core.py

@@ -155,13 +155,13 @@ class TestPytestPluginInteractions:
 
         config.pluginmanager.register(A())
         assert len(l) == 0
-        config.do_configure()
+        config._do_configure()
         assert len(l) == 1
         config.pluginmanager.register(A())  # leads to a configured() plugin
         assert len(l) == 2
         assert l[0] != l[1]
 
-        config.do_unconfigure()
+        config._ensure_unconfigure()
         config.pluginmanager.register(A())
         assert len(l) == 2