added ikev2/nat-virtual-ip scenario

testing/tests/ikev2/nat-virtual-ip/description.txt

@@ -0,0 +1,6 @@
+The router <b>moon</b> sets up a connection to gateway <b>sun</b> in order
+to reach the subnet hidden behind <b>sun</b>. The gateway <b>sun</b> assigns a
+virtual IP address to router <b>moon</b>. A special updown script on <b>moon</b>
+specified by <b>leftupdown=/etc/nat_updown</b> dynamically inserts a source NAT rule
+which maps the IP address of client <b>alice</b> to the virtual IP of <b>moon</b>.
+This allows <b>alice</b> to access client <b>bob</b> via the established IPsec tunnel.