support segmentation_id for vlan provider network

support segmentation_id for vlan provider network

If a network specified in the test case is a vlan provider network, new attribute "segmentation_id" can be used to specify the desired vlan tag in the test case network section:

networks:
  test-net:
    cidr: '192.168.1.0/24'
    provider: "vlan"
    physical_network: 'physnet1'
    segmentation_id: "1000"

If the "segmentation_id" attribute is absent, a random vlan tag will be allocated to the network.

Change-Id: Ic53852447a3c1bd8feb9ebd42d35f1ade3684be1
Signed-off-by: JingLu5 <lvjing5@huawei.com>