Handle empty DNS condition in pre-start powershell for windows

[mwright-pivotal]

At line 53 of consul-release/jobs/consul_agent_windows/templates/pre-start.ps1.erb, need to first check length of array before checking value. In some cases when DNS hadn't been set properly on the VM, the array may be empty causing the script to fail with NullArray error.

Example: PS C:\var\vcap\sys\log\consul_agent_windows> type pre-start.stderr.log
Cannot index into a null array.
At C:\var\vcap\jobs\consul_agent_windows\bin\pre-start.ps1:53 char:4

• if($servers[0] -eq "127.0.0.1")

• 
  + CategoryInfo          : InvalidOperation: (:) [], ParentContainsErrorRec                            
  ordException                                                                                          
  + FullyQualifiedErrorId : NullArray      

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[natalieparellano]

We (garden-windows) are also seeing this in one of our AWS deployments.

We think the issue is with these lines: https://github.com/cloudfoundry-incubator/consul-release/blob/5e78ece3b0eb2fc915cb3de121dbc91c90608d93/jobs/consul_agent_windows/templates/pre-start.ps1.erb#L45-L46

In our deployment that is failing, the first interface happens to be a Hyper-V virtual interface, which does not have any DNS servers associated with it, resulting in a null array.

Instead of grabbing the first interface, we should grab the interface that corresponds with the cell's IP.

For example, the following Powershell will return the DNS servers that we care about:

$ifindex = (Get-NetIPAddress | where {$_.IPAddress -eq  <cell ip>}).InterfaceIndex
Get-DnsClientServerAddress -InterfaceIndex $ifindex -AddressFamily ipv4

cc @sesmith177

[evanfarrar]

This is fixed in #89 and released in v195.