Use a predefined templates for default configs for haproxy config

Right now the HAPROXY configuration entries are all generated at run time. This way the configuration items are all fixed and there will be times when a deployer wants additional/less default configuration items.
We can have a default configuration file whose path will be put in the contrail-vrouter-agent.conf which has the necessary configuration items. Except for the items that are configures by the client of opencontrail like some configuration entries for frontend and backend, all the other entries are replaced by the items present in this default configuration file