Restoring openstack_release option

This patch restores openstack_release option in globals.yml

Closes-Bug: 1565353

Change-Id: I00342d1f52360e7d2c38b4467dc0b4049b7191dd

etc/kolla/globals.yml

@@ -16,6 +16,9 @@
 # Valid options are [ binary, source ]
 #kolla_install_type: "binary"
 
+# Valid option is Docker repository tag
+#openstack_release: "3.0.0"
+
 # This should be a VIP, an unused IP on your network that will float between
 # the hosts running keepalived for high-availability. When running an All-In-One
 # without haproxy and keepalived, this should be the first IP on your