Allow creating ec2 security groups without group owner id

[moretea]

According to the documentation, the parameter SourceSecurityGroupOwnerId (CTRL+F for it) is not required.

However, it appears that if you pass a sourceGroup.groupName to a security group (snippet below), that nixops just throws an error error: 'NoneType' object has no attribute 'get'

  resources.ec2SecurityGroups.zk-cluster-security-group-member = { 
    inherit region;
    rules = [ 
      { fromPort = 2888; toPort = 2888; sourceGroup.groupName = "zk-member"; }
      { fromPort = 3888; toPort = 3888; sourceGroup.groupName = "zk-member"; }
    ];  
  };