Install chrony in telegraf image

The output of "chronyc tracking"  command is required by
the inputs.chrony plugin.

Change-Id: I8fbef57b99122b8a093150f6ca1efdd33e032917

docker/telegraf/Dockerfile.j2

@@ -8,6 +8,7 @@ LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build
 {{ macros.configure_user(name='telegraf', homedir='/etc/telegraf') }}
 
 {% set telegraf_packages = [
+    'chrony',
     'net-tools',
     'python-pip',
     'telegraf'