[microbosh] If a public IP cannot be discovered, fall back to configured vip/ip [fixes #558]

The following micro_bosh.yml snippet previously failed unnecessarily:

    network:
      type: manual
      ip: 10.15.212.70
      cloud_properties:
        subnet: subnet-0420752c

Now, the network.ip will be used as the client_services_ip.

The following is output in the logs:

    using configured ip=10.15.212.70
Fix a BAT dns value error: the nameserver should come from env.dns_host
rather than env.director