[LIBCLOUD-587] ex_modify_network_attributes method

http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-ModifyVpcAttribute.html

This is kinda weird EC2 method.
The attributes cannot be modified in the same request:

*** Exception: InvalidParameterCombination: Fields for multiple attribute types specified: enableDnsSupport, enableDnsHostnames

So I had to make two requests.