T652: SNMP: bugfix for default auth and priv type
T652: SNMP: bugfix for default auth and priv type

If no auth or no priv type was specified for a v3 user this resulted in a commit
error, but CLI said there would be ssane defaults.

* auth type defaults to 'md5'
* priv type defaults to 'des'

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x