Pull cord-tester images from dockerhub as they are available now on dockerhub.
Change image names to be compatible with dockerhub image names.
Update prerequisites.sh last step to pull all the cord-tester docker images.
This should make the cord-test.py build all step redundant now since images should be available for testing immediately after prerequisites.sh are run.

Change-Id: I64cf664a214924349d6728ca2a52974decdef47f
diff --git a/src/test/setup/prerequisites.sh b/src/test/setup/prerequisites.sh
index 1cbea42..4874838 100755
--- a/src/test/setup/prerequisites.sh
+++ b/src/test/setup/prerequisites.sh
@@ -117,3 +117,9 @@
     service openvswitch-switch stop
     install_ovs
 fi
+
+test_images=(cordtest/radius:candidate cordtest/quagga:candidate cordtest/nose:candidate)
+for img in ${test_images[*]}; do
+    echo "Pulling cord-tester image $img"
+    docker pull $img 2>/dev/null
+done