VCNS driver implementation

Implement API/driver interface for configuring vShield Edge Appliance.

Currently implemented functions:
    - Deploy an Edge
    - Destroy an Edge
    - Configuring interfaces
    - Configuring SNAT/DNAT rules
    - Configuring default gateway and static routes
    - Query Edge status
    - Task-based asynchronous model
    - Allow old routes/nat config to be skipped if new updates are coming

Implements: blueprint vcns-driver
Change-Id: I881bde907f4c90de4c919d008b76b8c2a2d0e1fd
Merge "VCNS driver implementation"