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

ipalib/constants.py

@@ -188,6 +188,7 @@ DEFAULT_CONFIG = (
     ('confdir', object),  # Directory containing config files
     ('conf', object),  # File containing context specific config
     ('conf_default', object),  # File containing context independent config
+    ('plugins_on_demand', object),  # Whether to finalize plugins on-demand (bool)
 
     # Set in Env._finalize_core():
     ('in_server', object),  # Whether or not running in-server (bool)