gtpu: encap-vrf-id in show gtpu tunnels command

Show encap-vrf-id instead of fib-idx.  When a tunnel is created, you
specify encap-vrf-id, hence it makes sense for the show command to
present it the same way.  Fib index, on the other hand, is the internal
artifact of how IP tables are managed; showing it isn't helpful.

Type: improvement
Change-Id: I8d1713d6727dd8685d6c10c07f421c18edafa6e0
Signed-off-by: Nick Zavaritsky <nick.zavaritsky@emnify.com>