VPP-25 Add API for GRE tunnel create/delete/show.

Add API methods to create, delete and show GRE tunnels.

Also add missing CLI functionality for deleting and listing
configured tunnels.

Change-Id: I7565966037d94ade07938e4ff0d9333419716857
Signed-off-by: Chris Luke <chrisy@flirble.org>
6 files changed