Add IPSec interface FIB index for TX packet

This patch adds a configuration parameter to IPSec tunnels, enabling
custom FIB selection for encapsulated packets.
Although this option could also be used for policy-based IPSec,
this change only enables it for virtual-tunnel-interface mode.

Note that this patch does change the API default behavior regarding
TX fib selection for encapsulated packets.
Previous behavior was to use the same FIB after and before encap.
The new default behavior consists in using the FIB 0 as default.

Change-Id: I5c212af909940a8cf6c7e3971bdc7623a2296452
Signed-off-by: Pierre Pfister <ppfister@cisco.com>