Inject "wim_port_mappings" from "wim.config"

Automatically create/update port mappings during WIM creation/update,
as discussed in the WIM implementation meeting (12/Dec/2018).

Additionally fix small related errors and ensure wan_port_mappings
are returned by the HTTP server using the same keys forming the schema.

Change-Id: Icc27ad85c1de826ed96cb42b377055ea1b0c2cab
Signed-off-by: Anderson Bravalheri <a.bravalheri@bristol.ac.uk>