AWS requires Interface to be specified when using elasticIP

## Feature Request

When setting an elastic IP for an AWS instance that is using multiple interfaces you must specify the interface you wish to use with the elasticIP.

Currently the method does not allow you to specify the interface: ex_associate_address_with_node(node, elastic_ip).

I propose adding an optional parameter to this function that would allow you to define the interface.

Such as: ex_associate_address_with_node(node, elastic_ip, interface).

Start by locating ex_associate_address_with_node in the Python provider implementation and inspect how elastic IP association is currently requested. Trace the AWS API parameters for selecting an interface, then update the relevant tests to cover the optional interface argument. Done means existing two-argument calls still work and callers can associate the elastic IP with a chosen interface.

Written by the indexing model from the issue text.