Heat: support vnic_type in heat type context

Heat: support vnic_type in heat type context

JIRA: YARDSTICK-757

Some test scenarios require VM with specific vnic type.
This work is about supporting using different vnic types in heat type context.

context:
  name: demo
  image: cirros-0.3.5
  flavor: yardstick-flavor
  user: cirros

  placement_groups:
    pgrp1:
      policy: "availability"

  servers:
    athena:
      floating_ip: true
      placement: "pgrp1"
    ares:
      placement: "pgrp1"

  networks:
    test:
      cidr: '10.0.1.0/24'
      vnic_type: "normal"

Change-Id: Ia229fda72f47b04288ea107e2d58fd3e8ac91dd9
Signed-off-by: JingLu5 <lvjing5@huawei.com>