Simplify Openstack services configuration possibilities

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, all not documented:
- /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 remove them.

Documentation has been added for overriding configuration globally and
release note has been added too.

Closes-Bug: #1682479
Change-Id: I5d922dfc0d938173bad34ac64e490b78db1b7e31