[microbosh] fall back to configured network.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