Updated models, modeling existing SAL and its functionality

model-flow-base
  - match-types.yang - Match types categorized by protocol / layer
  - flow-types.yang - Base flow types / actions
model-flow-service
  - flow-service.yang - Model of Flow Programming service
model-flow-statistics
  - flow-statistics.yang - Flow Statistics service
model-inventory
  - model-inventory.yang - Device inventory

Change-Id: I14e9fe3c7a85e49131e102cf4c840bb1bdb0afbc
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>