[doc] Update CONFIG_DB manual to include NTP global settings (#1741)

Updated the Configuration Database documentation to include settings supported by the NTP daemon to globally configure its source port, which controls the port from which NTP requests are made (needs to be overridden in some cases) and also controls whether it runs within the mgmt vrf (which also needs to be set in some common configurations)

Author: alexrallen

doc/Configuration.md

@@ -30,6 +30,7 @@ Table of Contents
          * [Management port](#management-port)  
          * [Management VRF](#management-vrf)  
          * [MAP_PFC_PRIORITY_TO_QUEUE](#map_pfc_priority_to_queue)  
+         * [NTP Global Configuration](#ntp-global-configuration)  
          * [NTP and SYSLOG servers](#ntp-and-syslog-servers)  
          * [Port](#port)   
          * [Port Channel](#port-channel)  
@@ -915,7 +916,48 @@ instead of data network.
   }
 }
 ```
+### NTP Global Configuration
 
+These configuration options are used to modify the way that
+ntp binds to the ports on the switch and which port it uses to
+make ntp update requests from.
+
+***NTP VRF***
+
+If this option is set to `default` then ntp will run within the default vrf
+**when the management vrf is enabled**. If the mgmt vrf is enabled and this value is
+not set to default then ntp will run within the mgmt vrf.
+
+This option **has no effect** if the mgmt vrf is not enabled.
+
+```
+{
+"NTP": {
+    "global": {
+        "vrf": "default"
+        }
+    }
+}
+```
+
+
+***NTP Source Port***
+
+This option sets the port which ntp will choose to send time update requests from by.  
+
+NOTE: If a Loopback interface is defined on the switch ntp will choose this by default, so this setting
+is **required** if the switch has a Loopback interface and the ntp peer does not have defined routes
+for that address.
+ 
+```
+{
+"NTP": {
+    "global": {
+        "src_intf": "Ethernet1"
+        }
+    }
+}
+```
 
 ### NTP and SYSLOG servers