Contrailen 174 - Provide a mechanism to specify when host routes should NOT be summarized

Default treatment of Contrail advertised VM host routes will be to summarize them at the WAN boundary.
We require the ability to announce host routes to specific VMs. This request is for Contrail to provide a BGP community based mechanism to specify that a specific VM IP address should be advertised to the WAN as-is (e.g a host route).
This request requires the ability to specify a community value via Heat template that will be associated with a VM IP addressed and advertised by Contrail.
The objective is to, identify based on community value, source routes that must be propagated from the WAN boundary to Contrail. The notion is source routes originated from a Contrail vRouter can be stamped with an assigned CV. The CV will be used within a route policy that will allow paths with the CV to propagate to Contrail whereas paths without the CV will rely on an aggregate prefix.