OS-521 Fix router-gateway-set

router-gateway-set failed with cryptic error message 'KeyError: 'id''.
This was caused by port not having 'id' associated with it.

Change-Id: I2257d64e63e5a2f08adf2434a05c72e55182fb3c
Merge pull request #3 from roman-verchikov/bp/infoblox-plugin-neutron

OS-521 Fix router-gateway-set