Refactor test_type_gre/vxlan to reduce duplicate code

gre and vxlan type drivers have similar implementation for multiple
methods and associated tests. This change abstracts these tests.

Change-Id: Ic0e2119a81540c509c214008cce9dc50f55dd6de
Apic drivers enhancements (second approach): Backend

- Move backend facing modules to a library

Implements blueprint: apic-driver-enhancements

Change-Id: Ia4553d3885ca1886ca6bc7ea5d96afeace5fc457
Merge "Refactor test_type_gre/vxlan to reduce duplicate code"
Merge "Apic drivers enhancements (second approach): Backend"