title: List issue during upgrade from Terraform 0.11 to 0.12
body: <p>I have a problem trying to rewrite a list to work in Terraform 0.12
  that works in Terraform 0.11.</p>
  
  <p>This is my list:</p>
  
  <pre><code>variable "master_authorized_networks_config" {
    type        = list(string)
    description = "The list of CIDR blocks of master authorized networks."
  
    default = [
      {
        cidr_blocks = [
          { 
            cidr_block   = "XXXXXX/32"
            display_name = "XXXXX"
          },
          { 
            cidr_block   = "XXXXXX/32"
            display_name = "XXXXX"
          },
          { 
            cidr_block   = "XXXXXX/32"
            display_name = "XXXXX"
          },
          { 
            cidr_block   = "XXXXXX/32"
            display_name = "XXXXX"
          },
        ]
      },
    ]
  </code></pre>
  
  <p>I added it to module:
  <code>master_authorized_networks_config = var.master_authorized_networks_config</code></p>
  
  <p>After running <code>terraform apply</code> I get the following error:</p>
  
  <pre><code>Error: Invalid default value for variable
  
    on ../../modules/xxx/xxx/variables.tf line 71, in variable "master_authorized_networks_config":
    71:   default = [
    72:     {
    73:       cidr_blocks = [
    74:         {
    75:           cidr_block   = "XXXXXX/32"
    76:           display_name = "XXXXX"
    77:         },
    78:         {
    79:           cidr_block   = "XXXXXX/32"
    80:           display_name = "XXXXX"
    81:         },
    82:         {
    83:           cidr_block   = "XXXXXX/32"
    84:           display_name = "XXXXX"
    85:         },
    86:         {
    87:           cidr_block   = "XXXXXX/32"
    88:           display_name = "XXXXX"
    89:         },
   102:       ]
   103:     },
   104:   ]
  
  This default value is not compatible with the variable's type constraint:
  element 0: string required.
  </code></pre>
  
  <p>I can't resolve this problem. Can you help me?</p>