Fix up FROM line in galera

While the existing FROM line is technically correct, prefer we
use jinja tmeplating for the whole thing, so that we can remove
the special casing of the build.py script once the docker dir
goes.

Change-Id: I27efcd55f1b45d32e4504734edf9e1a305696ac1
Partially-Implements: blueprint dockerfile-templates

docker_templates/galera/Dockerfile.j2

@@ -1,4 +1,4 @@
-FROM %%KOLLA_NAMESPACE%%/%%KOLLA_PREFIX%%base:%%KOLLA_TAG%%
+FROM {{ namespace }}/{{ base_distro }}-{{ install_type }}-base:{{ tag }}
 MAINTAINER Kolla Project (https://launchpad.net/kolla)
 
 {% if base_distro in ['fedora', 'centos', 'oraclelinux'] %}