flow: code refactor

flow: code refactor

This is the code refactor for vnet/flow infra and the dpdk_plugin flow
implementation. The main works of the refactor are:
1. Added two base flow type: VNET_FLOW_TYPE_IP4 and VNET_FLOW_TYPE_IP6
   as the base the flow type
2. All the other flows are derived from the base flow types
3. Removed some flow types that are not currently supported by
   the hardware, and VPP won't leverage them either:
   IP4_GTPU_IP4, IP4_GTPU_IP6, IP6_GTPC, IP6_GTPU,
   IP6_GTPU_IP4, IP6_GTPU_IP6
4. Re-implemented the vnet/flow cli as well as the dpdk_plugin
   implementation
5. refine cli prompt
6. refine display info in command "show flow entry"

Type: refactor

Signed-off-by: Chenmin Sun <chenmin.sun@intel.com>
Change-Id: Ica5e61c5881adc73b28335fd83e36ec1cb420c96