refresh the devstack plugin docs, add plugin registry

The devstack plugin docs mostly referred to in tree plugins, which is
honestly something we don't want people doing. Instead restructure the
whole document to talk about external plugins as the only kinds of
plugins, and focus on a workflow to make that easy for people to work
through.

This also adds a plugin-registry page to start listing known plugins
somewhere centrally. Some sample content was added, hopefully people
will submit patches to include their plugins.

This does drop the section on hypervisor plugins. That's not currently
something that we expect a ton of people to work on, so diving into
the code for this should be fine.

Change-Id: Ifc0b831c90a1a45daa507a009d1dcffcd6e2deca
diff --git a/doc/source/plugin-registry.rst b/doc/source/plugin-registry.rst
new file mode 100644
index 0000000..2dd70d8
--- /dev/null
+++ b/doc/source/plugin-registry.rst
@@ -0,0 +1,73 @@
+..
+  Note to reviewers: the intent of this file is to be easy for
+  community members to update. As such fast approving (single core +2)
+  is fine as long as you've identified that the plugin listed actually exists.
+
+==========================
+ DevStack Plugin Registry
+==========================
+
+Since we've created the external plugin mechanism, it's gotten used by
+a lot of projects. The following is a list of plugins that currently
+exist. Any project that wishes to list their plugin here is welcomed
+to.
+
+Official OpenStack Projects
+===========================
+
+The following are plugins that exist for official OpenStack projects.
+
++--------------------+-------------------------------------------+--------------------+
+|Plugin Name         |URL                                        |Comments            |
++--------------------+-------------------------------------------+--------------------+
+|magnum              |git://git.openstack.org/openstack/magnum   |                    |
++--------------------+-------------------------------------------+--------------------+
+|trove               |git://git.openstack.org/openstack/trove    |                    |
++--------------------+-------------------------------------------+--------------------+
+|zaqar               |git://git.openstack.org/openstack/zarar    |                    |
++--------------------+-------------------------------------------+--------------------+
+
+
+
+Drivers
+=======
+
++--------------------+-------------------------------------------------+------------------+
+|Plugin Name         |URL                                              |Comments          |
++--------------------+-------------------------------------------------+------------------+
+|dragonflow          |git://git.openstack.org/openstack/dragonflow     |[d1]_             |
++--------------------+-------------------------------------------------+------------------+
+|odl                 |git://git.openstack.org/openstack/networking-odl |[d2]_             |
++--------------------+-------------------------------------------------+------------------+
+
+.. [d1] demonstrates example of installing 3rd party SDN controller
+.. [d2] demonstrates a pretty advanced set of modes that that allow
+        one to run OpenDayLight either from a pre-existing install, or
+        also from source
+
+Alternate Configs
+=================
+
++-------------+------------------------------------------------------------+------------+
+| Plugin Name | URL                                                        | Comments   |
+|             |                                                            |            |
++-------------+------------------------------------------------------------+------------+
+|glusterfs    |git://git.openstack.org/stackforge/devstack-plugin-glusterfs|            |
++-------------+------------------------------------------------------------+------------+
+|             |                                                            |            |
++-------------+------------------------------------------------------------+------------+
+
+Additional Services
+===================
+
++-------------+------------------------------------------+------------+
+| Plugin Name | URL                                      | Comments   |
+|             |                                          |            |
++-------------+------------------------------------------+------------+
+|ec2-api      |git://git.openstack.org/stackforge/ec2api |[as1]_      |
++-------------+------------------------------------------+------------+
+|             |                                          |            |
++-------------+------------------------------------------+------------+
+
+.. [as1] first functional devstack plugin, hence why used in most of
+         the examples.