Correct FROM line in openvswitch-agent

Missing KOLLA_TAG variable

Change-Id: Ic97841cb9d27a9fc5cb7967f3da1b2f016988d20
Closes-bug: #1469332

docker/centos/binary/neutron/neutron-openvswitch-agent/Dockerfile

-FROM %%KOLLA_NAMESPACE%%/%%KOLLA_PREFIX%%neutron-base
+FROM %%KOLLA_NAMESPACE%%/%%KOLLA_PREFIX%%neutron-base:%%KOLLA_TAG%%
 MAINTAINER Kolla Project (https://launchpad.net/kolla)
 
 # Install required packages