teib: Rename NHRP to TEIB

Type: refactor

The Tunnel Endpoint Informatiob Base (TEIB) is a better
description of what it is (a mapping between tunnel endpoint
address, in the overlay, and next-hop address, in the underlay)
whereas NHRP is one instanc eof a control protocol that might add
such endpoints.

Signed-off-by: Neale Ranns <nranns@cisco.com>
Change-Id: Idcb2ad0b6543d3e5d9f6e96f9d14dafb5ce2aa85