Iterating over a deferred value in a for loop generates an invalid ARM template

[anthony-c-martin]

Created from discussion #2072

The following doesn't work (gives template function 'reference' is not expected at this location at deploy time):

output snetIds array = [for subnet in vnet.properties.subnets: {
  subName: subnet.name
  subId: subnet.id
}]

Instead, the following must be used:

output snetIds array = [for i in range(0,length(subnets)): {
  subName: vnet.properties.subnets[i].name
  subId: vnet.properties.subnets[i].id
}]

[majastrz]

This is somewhat related to #2391