Set empty openstack_release option

globals.yml have the highest var precedence in Ansible vars
as is added in kolla-ansible as -e @globals.yml
Setting openstack_release: "auto" causes error while
deploying/pulling as this var override the fact set in site.yml.

Value in globals.yml should be a real kolla version
(3.0.0, 4.0.0, etc)
Value in group_vars/all works fine with auto.

Change-Id: If99f7b4b1507909c6c2628b6d6fc6ce2519c9a24
Closes-Bug: #1671143

etc/kolla/globals.yml

@@ -18,7 +18,7 @@
 #kolla_install_type: "binary"
 
 # Valid option is Docker repository tag
-#openstack_release: "auto"
+#openstack_release: ""
 
 # Location of configuration overrides
 #node_custom_config: "/etc/kolla/config"