Add warning for enabled_apis='ec2' in release notes

As was discovered in IRC, many deployment tools (such as packstack)
hardcode the value of enabled_apis. This edge case was not handled in
our ec2 upgrade path, and nova-api will die if that value is
there. Add a warning about that.

Change-Id: Iae4b05d9f127791a6efc6f7ee3b1053082d05798

releasenotes/notes/remove_ec2_and_objectstore_api-4ccb539db1d171fa.yaml

@@ -1,7 +1,14 @@
 ---
 upgrade:
-  - All code and tests for Nova's EC2 and ObjectStore API support which
+  - |
+    All code and tests for Nova's EC2 and ObjectStore API support which
     was deprecated in Kilo
     (https://wiki.openstack.org/wiki/ReleaseNotes/Kilo#Upgrade_Notes_2) has
     been completely removed in Mitaka. This has been replaced by the new
-    ec2-api project (http://git.openstack.org/cgit/openstack/ec2-api/).
+    ec2-api project (http://git.openstack.org/cgit/openstack/ec2-api/).
+
+    .. warning:: Some installation tools (such as ``packstack``) hardcode the
+      value of ``enabled_apis`` in your nova.conf. While the defaults
+      for ``enabled_apis`` dropped ``ec2`` as a value, if that is hard
+      coded in your nova.conf, you will need to remove it before
+      restarting Nova's API server, or it will not start.