Add cni config template support.

Signed-off-by: Lantao Liu <lantaol@google.com>

docs/config.md

@@ -63,6 +63,13 @@ The explanation and default value of each configuration item are as follows:
     # conf_dir is the directory in which the admin places a CNI conf.
     conf_dir = "/etc/cni/net.d"
 
+    # conf_template is the file path of golang template used to generate
+    # cni config.
+    # If this is set, containerd will generate a cni config file from the
+    # template. Otherwise, containerd will wait for the system admin or pod
+    # network addon to drop the config file into the conf_dir.
+    conf_template = ""
+
   # "plugins.cri.registry" contains config related to the registry
   [plugins.cri.registry]