ipalib, ipaserver: fix incorrect API.register calls in docstrings

Use API.add_plugin to load specific plugins into API objects. Use Registry
to register plugins.

This fixes doctests.

https://fedorahosted.org/freeipa/ticket/4739
https://fedorahosted.org/freeipa/ticket/5115

Reviewed-By: David Kupka <dkupka@redhat.com>

ipalib/crud.py

@@ -56,9 +56,9 @@ Now we'll register the plugins and finalize the `plugable.API` instance:
 
 >>> from ipalib import create_api
 >>> api = create_api()
->>> api.register(user)
->>> api.register(user_add)
->>> api.register(user_show)
+>>> api.add_plugin(user)
+>>> api.add_plugin(user_add)
+>>> api.add_plugin(user_show)
 >>> api.finalize()
 
 First, notice that our ``user`` `Object` has the params we defined with the