Add lifecycle ignore on az's

[myoung34]

If you leave azs parameter off, it chooses the minimum which is great. However, any subsequent runs show a delete/create since the module wants to set to 0 but there are >1 set by the API:

-/+ module.db.aws_rds_cluster.default (new resource required)                                              
      id:                                  "aurora-cluster" => <computed> (forces new resource)                             
      apply_immediately:                   "true" => "true"                                                                             
      availability_zones.#:                "3" => "0" (forces new resource)                                                          
      availability_zones.1305112097:       "us-east-1b" => "" (forces new resource)                        
      availability_zones.3551460226:       "us-east-1e" => "" (forces new resource)                                                       
      availability_zones.3569565595:       "us-east-1a" => "" (forces new resource)                                                                    
      backup_retention_period:             "5" => "5"