Added check for fixed_ip in  add_router_interface

Change-Id: I255e7eda5aa1b7b4644014386ae66f1ebd7c732f
Fix: #29
Signed-off-by: Rossella Sblendido <rossella@midokura.com>
Merge pull request #30 from midokura/issue29

Added check for fixed_ip in  add_router_interface