Allow API plugin registration via a decorator

This makes plugin registration easier to read, less error-prone, and,
for many Plugins in a single module, faster to write.

Functionally, the decorator is equivalent to current plugin
registration. However, in the future this style will allow cleaner
semantics.

As an example, and to exercise the new syntax to prevent regressions,
the ping plugin is converted to this style.

ipalib/plugable.py

@@ -59,6 +59,33 @@ def is_production_mode(obj):
     return obj.env.mode == 'production'
 
 
+class Registry(object):
+    """A decorator that makes plugins available to the API
+
+    Usage::
+
+        register = Registry()
+
+        @register()
+        class obj_mod(...):
+            ...
+
+    For forward compatibility, make sure that the module-level instance of
+    this object is named "register".
+    """
+    # TODO: Instead of auto-loading when plugin modules are imported,
+    # plugins should be stored in this object.
+    # The API should examine it and load plugins explicitly.
+    def __call__(self):
+        from ipalib import api
+
+        def decorator(cls):
+            api.register(cls)
+            return cls
+
+        return decorator
+
+
 class SetProxy(ReadOnly):
     """
     A read-only container with set/sequence behaviour.

ipalib/plugins/ping.py

@@ -17,10 +17,10 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-from ipalib import api
 from ipalib import Command
 from ipalib import output
 from ipalib import _, ngettext
+from ipalib.plugable import Registry
 from ipapython.version import VERSION, API_VERSION
 
 __doc__ = _("""
@@ -51,6 +51,10 @@ EXAMPLES:
    -----------------------------------------------------
 """)
 
+register = Registry()
+
+
+@register()
 class ping(Command):
     __doc__ = _('Ping a remote server.')
 
@@ -64,5 +68,3 @@ class ping(Command):
         back but a fixed value works for now.
         """
         return dict(summary=u'IPA server version %s. API version %s' % (VERSION, API_VERSION))
-
-api.register(ping)