Parameterizes webhook service URL as env variable

This change is useful since the previous webhook URL was hardcoded and was useless (since its value always change). To generate a new VNFD descriptor with this parametrization, just run: WEBHOOK_URL=<some_url> envsubst < cirros_alarm_vnf/cirros_alarm_vnfd.yaml > new_vnfd.yaml

Signed-off-by: garciaale agarcia@whitestack.com