Change Docker images pull strategy to PULL_LATEST

When running the Gerrit-CI on a ephemeral infrastructure
do not assume that images already exist: pull the latest
one from DockerHub so that any restart would allow to
fetch the images on-demand.

Change-Id: Iaac36e1d456a90f393d31f5f8138b69dbeef1f11

jenkins-docker/server/config.xml

diff --git a/jenkins-docker/server/config.xml b/jenkins-docker/server/config.xml
index 818355c..9eeb589 100644
--- a/jenkins-docker/server/config.xml
+++ b/jenkins-docker/server/config.xml
@@ -72,7 +72,7 @@
             </extraHosts>
           </dockerTemplateBase>
           <removeVolumes>false</removeVolumes>
-          <pullStrategy>PULL_NEVER</pullStrategy>
+          <pullStrategy>PULL_LATEST</pullStrategy>
         </com.nirima.jenkins.plugins.docker.DockerTemplate>
         <com.nirima.jenkins.plugins.docker.DockerTemplate>
           <configVersion>2</configVersion>
@@ -109,7 +109,7 @@
             </extraHosts>
           </dockerTemplateBase>
           <removeVolumes>false</removeVolumes>
-          <pullStrategy>PULL_NEVER</pullStrategy>
+          <pullStrategy>PULL_LATEST</pullStrategy>
         </com.nirima.jenkins.plugins.docker.DockerTemplate>
         <com.nirima.jenkins.plugins.docker.DockerTemplate>
           <configVersion>2</configVersion>
@@ -222,7 +222,7 @@
             </extraHosts>
           </dockerTemplateBase>
           <removeVolumes>false</removeVolumes>
-          <pullStrategy>PULL_NEVER</pullStrategy>
+          <pullStrategy>PULL_LATEST</pullStrategy>
         </com.nirima.jenkins.plugins.docker.DockerTemplate>
         <com.nirima.jenkins.plugins.docker.DockerTemplate>
           <configVersion>2</configVersion>
@@ -259,7 +259,7 @@
             </extraHosts>
           </dockerTemplateBase>
           <removeVolumes>false</removeVolumes>
-          <pullStrategy>PULL_NEVER</pullStrategy>
+          <pullStrategy>PULL_LATEST</pullStrategy>
         </com.nirima.jenkins.plugins.docker.DockerTemplate>
         <com.nirima.jenkins.plugins.docker.DockerTemplate>
           <configVersion>2</configVersion>