Fix issues to run in icehouse

This pull request fixes the changes required to run opencontrail plugin with neutron icehouse
Two changes are done
1. neutron icehouse has removed 'CAPABILITIES' and added 'VIF_DETAILS' in portbindings.py. Added the code to fix this
2. /opt/stack/neutron/neutron/db/db_base_plugin_v2.py in the function _make_subnet_dict, expects 'ipv6_ra_mode' and 'ipv6_address_mode' keys to be present in the 'subnet' dictionary. Added the code to fix this.