OSMENG-1062: Fix states in VnfInstantiateWorkflow

At the beginning of the workflow, update only VnfInstantiationState
(NOT_INSTANTIATED).
At the end of the workflow, update VnfInstantationState and VnfState:
* if succeeded: INSTANTIATED, STARTED
* if failed: INSTANTIATED, STOPPED

Change-Id: Ifb6df225d4e09f48941ce649ad3195d2b4e8b3b4
Signed-off-by: Dario Faccin <dario.faccin@canonical.com>