Implement NetworkInterfaces instance property.

This accepts a list of strings, where each string will be a Ref
to either a OS::Quantum::Port or a (yet to be implemented)
AWS::EC2::NetworkInterface.

This fixes bug 1096013

Change-Id: Ic8aecf8d59395edcf111b3a778e79762af0e50d4
Merge "Implement NetworkInterfaces instance property."