Support configuring domain name of the load balancer

[norshtein]

Solve (a part of?) #36 .
Support config domain name of the load balancer. Users can config the domain name by setting domain_name in the load balancer field in the manifest. An example:

load_balancers:
  - name: load_balancer_1
    sku: Standard
    backend_address_pools:
      - name: backend_address_pool_1
    domain_name: tositestdomain             ###############NOTE HERE###################
    health_probes:
      - name: http_probe
        protocol: Tcp # optional values: [Tcp, Http, Https]
        port: 80
        # request_path: "/" # required if protocol is set to Http or Https. Otherwise, it is not allowed.
      - name: ssh_probe
        protocol: Tcp
        port: 22
    rules:
      - name: http_rule
        protocol: Tcp # optional values: [Tcp, Udp, All]
        frontend_port: 80
        backend_port: 80
        backend_address_pool_name: backend_address_pool_1
        health_probe_name: http_probe

However, I'm not very clear about when to edit this field in our workflow. Obviously, we can't give a default value in buildinmanifests, since domain name should be unique across an Azure region. Maybe we should also generate a patch file and remind the user to fill in the patch file?
Feel free to contact me at any time.

CC: @andyliuliming

[bingosummer]

1. kl analyze should generate the domain name label to make it unique. I think, there's no need to ask users to input it. They can overwrite it manually. If users do care about it, they may want to use their custom domain, which may need Azure DNS zone. That's a different story. Wdyt? @norshtein @andyliuliming
2. @norshtein please add the domain name into the VM's network outputs in vm.go.