Update port fails with KeyError in case of ips update

[evgenyfedoruk]

When update port request comes with 'fixed_ips' attribute, meaning port ips are updated,
KeyError raised and request fails.

The issue is with the '_update_ips_for_port' function of NeutronPluginContrailCoreBase class in contrail_plugin_base module.
It accesses the cfg.CONF.max_fixed_ips_per_port parameter which no longer exists, since N release.
It should check the parameter existence first, like 'if hasattr(cfg.CONF, 'max_fixed_ips_per_port')'.