allow specifying standard as a domain in ex_associate_address_with_node

AWS uses "domain" field to distinguish between EIP allocated for Classic EC2 ('standard') and EIPs allocated for VPC (value 'vpc')

domain
    Indicates whether this Elastic IP address is for use with instances in EC2-Classic (standard) or instances in a VPC (vpc).
    Type: xsd:string
    Valid values: standard | vpc

http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-ItemType-DescribeAddressesResponseItemType.html

The method ex_allocate_address() also uses 'standard' and 'vpc' as allowed parameters.

This patch adds 'standard' as a value to domain parameter.
None is left for backward compatibility.