Add new fields to instance_net.vim_info

VIM Connectors should provide the fields "encapsulation_type" and
"encapsulation_id", as discussed in the WIM implementation meeting
(21/Nov/2018).

Change-Id: I654d8fa93bef7c37c7ce575c0335154e99fd7347
Signed-off-by: Anderson Bravalheri <a.bravalheri@bristol.ac.uk>