Enable configuration of contrail extensions

This pull request provides the option to enable or disable opecontrail extensiosn - ipam, policy and route-table.

The code is taken from the https://github.com/Juniper/neutron/blob/contrail/havana-R1.06/neutron/plugins/juniper/contrail/contrail_plugin_core.py

Since there is no file for each of the extensions (contrail_plugin_ipam.py etc) in this repo yet, the 'contrail_extensions' in the ContrailPlugin.ini is of the below format 
contrail_extensions=ipam,policy,...