Optionally wait for 389-ds postop plugins to complete

Add a new command that lets you wait for an attribute to appear in
a value. Using this you can do things like wait for a managed entry
to be created, adding a new objectclass to the parent entry.

This is controlled by a new booleon option, wait_for_attr, defaulting
to False.

https://fedorahosted.org/freeipa/ticket/1144

ipalib/constants.py

@@ -148,6 +148,7 @@ DEFAULT_CONFIG = (
     # Enable certain optional plugins:
     ('enable_ra', False),
     ('ra_plugin', 'selfsign'),
+    ('wait_for_attr', False),
 
     # Used when verifying that the API hasn't changed. Not for production.
     ('validate_api', False),