Validate expected parameters in add/remove router interfaces

The add and remove router interface methods check that interface_info
is not empty but don't check if it contains any of expected parameters:
port_id and subnet_id
This patch adds a helper method to validate that interface_info contains
at least one of the expected parameters
Include a unit test for the empty port_id and subnet_id case

Closes-Bug: #1325982

Change-Id: Ia370565235a33a847704b972c875d8f1573306c0
Merge "Validate expected parameters in add/remove router interfaces"