plugable: support plugin versioning

Allow multiple incompatible versions of a plugin using the same name. The
current plugins are assumed to be version '1'.

The unique identifier of plugins was changed from plugin name to plugin
name and version. By default, the highest version available at build time
is used. If the plugin is an unknown remote plugin, version of '1' is used
by default.

https://fedorahosted.org/freeipa/ticket/4427

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

ipalib/frontend.py

@@ -1258,6 +1258,8 @@ class Object(HasParam):
         namespace = self.api[name]
         assert type(namespace) is APINameSpace
         for plugin in namespace(): # Equivalent to dict.itervalues()
+            if plugin is not namespace[plugin.name]:
+                continue
             if plugin.obj_name == self.name:
                 yield plugin
 
@@ -1328,10 +1330,16 @@ class Attribute(Plugin):
     In practice the `Attribute` class is not used directly, but rather is
     only the base class for the `Method` class.  Also see the `Object` class.
     """
+    obj_version = '1'
+
     @property
     def obj_name(self):
         return self.name.partition('_')[0]
 
+    @property
+    def obj_full_name(self):
+        return self.obj.full_name
+
     @property
     def attr_name(self):
         prefix = '{}_'.format(self.obj_name)
@@ -1340,7 +1348,7 @@ class Attribute(Plugin):
 
     @property
     def obj(self):
-        return self.api.Object[self.obj_name]
+        return self.api.Object[self.obj_name, self.obj_version]
 
 
 class Method(Attribute, Command):