Add ceilometer to plugin registry

Ceilometer is now removed from devstack and only exists as a plugin
so it should be in the registry.

Unfortunately the length of the URL changed the table formatting so
the diff is much larger than the semantic change.

Change-Id: Ibe8e27e97294c2d13be8f22f41eea27775811eec
diff --git a/doc/source/plugin-registry.rst b/doc/source/plugin-registry.rst
index 0feaafb..85fd7cc 100644
--- a/doc/source/plugin-registry.rst
+++ b/doc/source/plugin-registry.rst
@@ -17,27 +17,29 @@
 
 The following are plugins that exist for official OpenStack projects.
 
-+--------------------+-------------------------------------------+--------------------+
-|Plugin Name         |URL                                        |Comments            |
-+--------------------+-------------------------------------------+--------------------+
-|aodh                |git://git.openstack.org/openstack/aodh     | alarming           |
-+--------------------+-------------------------------------------+--------------------+
-|gnocchi             |git://git.openstack.org/openstack/gnocchi  | metric             |
-+--------------------+-------------------------------------------+--------------------+
-|magnum              |git://git.openstack.org/openstack/magnum   |                    |
-+--------------------+-------------------------------------------+--------------------+
-|manila              |git://git.openstack.org/openstack/manila   | file shares        |
-+--------------------+-------------------------------------------+--------------------+
-|mistral             |git://git.openstack.org/openstack/mistral  |                    |
-+--------------------+-------------------------------------------+--------------------+
-|rally               |git://git.openstack.org/openstack/rally    |                    |
-+--------------------+-------------------------------------------+--------------------+
-|sahara              |git://git.openstack.org/openstack/sahara   |                    |
-+--------------------+-------------------------------------------+--------------------+
-|trove               |git://git.openstack.org/openstack/trove    |                    |
-+--------------------+-------------------------------------------+--------------------+
-|zaqar               |git://git.openstack.org/openstack/zaqar    |                    |
-+--------------------+-------------------------------------------+--------------------+
++------------------+---------------------------------------------+--------------------+
+|Plugin Name       |URL                                          |Comments            |
++------------------+---------------------------------------------+--------------------+
+|aodh              |git://git.openstack.org/openstack/aodh       | alarming           |
++------------------+---------------------------------------------+--------------------+
+|ceilometer        |git://git.openstack.org/openstack/ceilometer | metering           |
++------------------+---------------------------------------------+--------------------+
+|gnocchi           |git://git.openstack.org/openstack/gnocchi    | metric             |
++------------------+---------------------------------------------+--------------------+
+|magnum            |git://git.openstack.org/openstack/magnum     |                    |
++------------------+---------------------------------------------+--------------------+
+|manila            |git://git.openstack.org/openstack/manila     | file shares        |
++------------------+---------------------------------------------+--------------------+
+|mistral           |git://git.openstack.org/openstack/mistral    |                    |
++------------------+---------------------------------------------+--------------------+
+|rally             |git://git.openstack.org/openstack/rally      |                    |
++------------------+---------------------------------------------+--------------------+
+|sahara            |git://git.openstack.org/openstack/sahara     |                    |
++------------------+---------------------------------------------+--------------------+
+|trove             |git://git.openstack.org/openstack/trove      |                    |
++------------------+---------------------------------------------+--------------------+
+|zaqar             |git://git.openstack.org/openstack/zaqar      |                    |
++------------------+---------------------------------------------+--------------------+