fix(config): NSSF port number was not read from config file

Found this when porting the slicing scenario to HTTP/2

added Fix v1.5.1 labels

requested review from @tien-thinh.nguyen

assigned to @defosseu

mentioned in merge request oai-cn5g-fed!115 (merged)

OAI OAI-CN5G-AMF build (701): passed (https://jenkins-oai.eurecom.fr/job/OAI-CN5G-AMF/701/)

approved this merge request

mentioned in commit 5ac85d05

merged