fix missing ntp variable error
It fails with:
```
----------
ID: /etc/ntp.conf
Function: file.managed
Result: False
Comment: Unable to manage file: Jinja variable 'ntp' is undefined
Started: 11:58:05.396994
Duration: 37.777 ms
Changes:
```
diff --git a/ntp/files/ntp.conf b/ntp/files/ntp.conf
index a3778ba..7b23f00 100644
--- a/ntp/files/ntp.conf
+++ b/ntp/files/ntp.conf
@@ -20,7 +20,7 @@
restrict 127.0.0.1
restrict ::1
-{%- if ntp.mode7 %}
+{%- if client.get('mode7') %}
# mode7 is required for collectd monitoring
enable mode7
{%- endif %}