Finalize plugin initialization on demand.

This patch changes the way plugins are initialized. Instead of
finalizing all the plugins at once, plugins are finalized only after
they are accessed (currently applies to Command, Object and
Attribute subclasses, only in CLI by default).

This change provides significant performance boost, as only the
plugins that are actually used are finalized.

ticket 1336

tests/test_ipalib/test_frontend.py

@@ -942,7 +942,8 @@ class test_Object(ClassChecker):
                 parameters.Str('four', primary_key=True),
             )
         o = example3()
-        e = raises(ValueError, o.set_api, api)
+        o.set_api(api)
+        e = raises(ValueError, o.finalize)
         assert str(e) == \
             'example3 (Object) has multiple primary keys: one, two, four'