multus: entrypoint: disallow using cni version <= 0.3.1 with >= 0.4.0

When nested CNI version is 0.3.1 or less while the top level CNI version
is 0.4.0, error out. Version 0.3.1 or less doesn't support the CHECK
command. This closes issue #737.

images/entrypoint.sh

@@ -87,6 +87,24 @@ if ! type python3 &> /dev/null; then
 	alias python=python3
 fi
 
+function checkCniVersion {
+      cniversion_python_tmpfile=$(mktemp)
+      cat << EOF > $cniversion_python_tmpfile
+import json, sys
+
+def version(v):
+    return [int(x) for x in v.split(".")]
+
+top_level = sys.argv[2]
+with open(sys.argv[1], "r") as f:
+    nested = json.load(f)["cniVersion"]
+if version(nested) <= version("0.3.1") and version(top_level) >= version("0.4.0"):
+    msg = "Multus cni version is %s while master plugin cni version is %s"
+    print(msg % (top_level, nested))
+EOF
+      python $cniversion_python_tmpfile $1 $2
+}
+
 # Parse parameters given as arguments to this script.
 while [ "$1" != "" ]; do
     PARAM=`echo $1 | awk -F= '{print $1}'`
@@ -374,6 +392,12 @@ EOF
       MASTER_PLUGIN_LOCATION=$MULTUS_AUTOCONF_DIR/$MASTER_PLUGIN
       MASTER_PLUGIN_JSON="$(cat $MASTER_PLUGIN_LOCATION)"
       log "Using $MASTER_PLUGIN_LOCATION as a source to generate the Multus configuration"
+      CHECK_CNI_VERSION=$(checkCniVersion $MASTER_PLUGIN_LOCATION $CNI_VERSION)
+      if [ "$CHECK_CNI_VERSION" != "" ] ; then
+        error "$CHECK_CNI_VERSION"
+        exit 1
+      fi
+
       CONF=$(cat <<-EOF
         {
           $CNI_VERSION_STRING