Add note about not use docker on CentOS8
Add note about not use docker on CentOS8

Current docker-ce is not supported in CentOS8.
So we should add note which explain devstack for using kubernetes
does not work on CentOS8.

Change-Id: Ie452debd5721093013d78544ef8c4ca03b1a32cc

stackforge/tacker,openstack/tacker,stackforge/tacker,openstack/tacker,openstack/tacker