NetworkPool object vlan_start attribute returns None if vlan_start: 0 is specified in devops yaml

[Andrii Ostapenko]

If we are using "vlan_start: 0" for network pool in devops yaml, corresponding devops model will return None for vlan_start attribute: https://github.com/openstack/fuel-devops/blob/master/devops/models/network.py#L394-L400

Also tg fails with 400 from nailgun if "vlan_start: 0" is specified for private network pool: https://github.com/openstack/fuel-qa/blob/master/fuelweb_test/models/fuel_web_client.py#L3167-L3169