Fix the router-interface-add issue

Because of the merge - https://review.opencontrail.org/#/c/554/
'neutron router-interface-add' is failing.
This is because, neutron router-interface-add' creates a port
with 'subnet_id' in 'fixed_ips' attribute.

This patch fixes this issue.

Change-Id: Ide21c811aac90b68465113d289978f299afd99d7
Merge "Fix the router-interface-add issue" into R1.06