Change WAN Actions to use VIM config rules

Extract switch ID and port by default from
vim.config.external_connections, as discussed in the WIM implementation
meeting (21/Nov/2018).

Change-Id: I95cc5fb0d6ad2822ef4bd1b80e7652bfabf34120
Signed-off-by: Anderson Bravalheri <a.bravalheri@bristol.ac.uk>