Code drop: improvements, major cleanups and some fixes

• add ability to interconnect an EVI and a VRF
• extended API to specify OVS interfaces to VRFs (and support in bagpipe-rest-attach)
• refactored dataplane driver callflows
• support for GRE/MPLS
• support for advertizing use of GRE/MPLS or VXLAN in a RFC5512 BGP Tunnel Encap extended community
• logging improvements
• more unit tests
• tracker worker fixes
• many many cleanups