Add release note to deprecate some config files

Actually Openstack services configuration can be overriden using many
files:
- /etc/kolla/config/<< service name >>/<< config file >>
- /etc/kolla/config/<< service name >>/<<host>>/<< config file >>
- /etc/kolla/config/global.conf
- /etc/kolla/config/database.conf
- /etc/kolla/config/messaging.conf

Only per-service configuration is actually documented here:
https://github.com/openstack/kolla-ansible/blob/master/doc/advanced-configuration.rst#L164

Allowing to globally modify service configuration can be perform too,
but it can be done in 3 different manners:
- /etc/kolla/config/global.conf
- /etc/kolla/config/database.conf
- /etc/kolla/config/messaging.conf

database.conf and messaging.conf seems redundant with global.conf.
In order to simplify codebase it seems logical to deprecate them.

Change-Id: Ia632c207e4b0237ea813fcf53b44504b97a204e7

releasenotes/notes/deprecate-some-config-files-0ed8c75f6bb4aa26.yaml

+---
+deprecations:
+  - |
+    The following files are deprecated and should be removed during Queens cycle,
+    use /etc/kolla/config/global.conf instead:
+
+    * /etc/kolla/config/database.conf
+    * /etc/kolla/config/messaging.conf
+