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/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)