[FEED PARSER] [NTB NIFS] fixed metadata initialization
[FEED PARSER] [NTB NIFS] fixed metadata initialization

metadata initialization was failing because vocabularies are not
accessible yet when __init__ is called.

This patch fixes this by initializing metadata on first parsing, and
then using cached values.

SDNTB-496

superdesk/superdesk-ntb,petrjasek/superdesk-ntb,petrjasek/superdesk-ntb,petrjasek/superdesk-ntb,ioanpocol/superdesk-ntb,superdesk/superdesk-ntb,petrjasek/superdesk-ntb,superdesk/superdesk-ntb,ioanpocol/superdesk-ntb,ioanpocol/superdesk-ntb,superdesk/superdesk-ntb