Server Upgrade: specify order of plugins in update files

* add 'plugin' directive
* specify plugins order in update files
* remove 'run plugins' options
* use ldapupdater API instance in plugins
* add update files representing former PreUpdate and PostUpdate order of plugins

https://fedorahosted.org/freeipa/ticket/4904

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

ipalib/frontend.py

@@ -1371,7 +1371,7 @@ class Method(Attribute, Command):
 
 
 @register.base()
-class Updater(Method):
+class Updater(Plugin):
     """
     An LDAP update with an associated object (always update).
 
@@ -1397,8 +1397,8 @@ class Updater(Method):
     >>> api.Updater.my_update # doctest:+ELLIPSIS
     ipalib.frontend.my_update()
     """
-    def __init__(self):
-        super(Updater, self).__init__()
+    def execute(self, **options):
+        raise NotImplementedError('%s.execute()' % self.name)
 
     def __call__(self, **options):
         self.debug(