Historical services can violate naming rules

After a few attempts at retrofitting the newly-minted service type
naming rules to established services, we've discovered that's more
hassle than it's worth.

This change updates the guidelines to make it okay for historical
service types to keep their established names, while reinforcing that
new service types should conform to the stated rules.

Change-Id: I1f9d45ae1ccc0a397e2cc83dc8bb61a9b61971fe

README.rst

@@ -81,17 +81,21 @@ project. ``placement`` is secondary.
 Naming
 ======
 
-Service types should be:
+.. note:: Established service types need not be forcefully retrofitted
+          to conform to these guidelines. For this reason you may see
+          entries such as ``ec2-api`` (contains a digit) or
+          ``clustering`` (an action, not a thing).
 
-- English words
-- Match the regex ``^[a-z][a-z-]*[a-z]$``. As a special case, ``ec2-api`` is
-  also allowed.
-- Meaningful
+New service type names should:
+
+- Be English words
+- Match the regex ``^[a-z][a-z-]*[a-z]$``.
+- Be meaningful
 - Not use terms which are incredibly overloaded in OpenStack space
   (e.g. policy)
-- A thing, not an action (e.g. load-balancer, not load-balancing)
-- Singular instead of plural (e.g. image, not images)
-- Unversioned (e.g. volume, not volumev2)
+- Be a thing, not an action (e.g. load-balancer, not load-balancing)
+- Be singular instead of plural (e.g. image, not images)
+- Be unversioned (e.g. volume, not volumev2)
 
 Non Official Types in Service Catalog
 =====================================