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>
diff --git a/osm_ro/wim/errors.py b/osm_ro/wim/errors.py
index 16c53b5..ca8c2b7 100644
--- a/osm_ro/wim/errors.py
+++ b/osm_ro/wim/errors.py
@@ -178,3 +178,12 @@
         message += ('\nThe thread responsible for processing the actions have '
                     'suddenly stopped, or have never being spawned')
         super(WimAccountNotActive, self).__init__(message, http_code)
+
+
+class NoExternalPortFound(HttpMappedError):
+    """No external port associated to the instance_net"""
+
+    def __init__(self, instance_net):
+        super(NoExternalPortFound, self).__init__(
+            '{} uuid({})'.format(self.__class__.__doc__, instance_net['uuid']),
+            http_code=Not_Found)