iosxe_ntp requiring attribute for setting global source interface

Currently iosxe_ntp resource does not allow to set on a global level the ntp source interface like below sample configuration

```
ntp source Loopback0
```

Asking for an attribute in iosxe_ntp to be able to set the above.

This values are part of Cisco-IOS-XE-ntp

```
/native/ntp/ios-ntp:source
```