traffic_packets: add trailing $ to fix packet_rpc_netlogon_30

traffic_packets: add trailing $ to fix packet_rpc_netlogon_30

For `NetrServerPasswordSet2`, the 2nd arg `account_name` must end with a
$, otherwise windows will return an `Access Denied` error.

Use `creds.get_username()` instead of `creds.get_workstation()` to
include the trailing $.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>