Replace JENKINS_DOCKER_REGISTRY with PRIVATE_CONTAINER_REGISTRY

On the following jobs
tf-a-builder-tfa-next.yaml
tf-a-builder.yaml
tf-a-lts2.10-builder.yaml
tf-a-lts2.8-builder.yaml

Change-Id: I326fc13d1fe28aa7607e3dfe67b5d18517d0dc2a
Signed-off-by: Arthur She <arthur.she@linaro.org>
diff --git a/tf-a-lts2.8-builder.yaml b/tf-a-lts2.8-builder.yaml
index bb326e7..07c1136 100644
--- a/tf-a-lts2.8-builder.yaml
+++ b/tf-a-lts2.8-builder.yaml
@@ -52,8 +52,8 @@
               tf-a-job-configs refspec to use. The master branch is used by default.
         - string:
             name: DOCKER_REGISTRY
-            default: ${JENKINS_DOCKER_REGISTRY}
-            description: JENKINS_DOCKER_REGISTRY is a system-wide environment variable
+            default: ${PRIVATE_CONTAINER_REGISTRY}
+            description: PRIVATE_CONTAINER_REGISTRY is a system-wide environment variable
         - string:
             name: JUNO_ROOTFS_URL
             default: 'http://releases.linaro.org/openembedded/aarch64/17.01/linaro-image-minimal-genericarmv8-20170127-888.rootfs.tar.gz'