app/testpmd: add flow command parsing routine

app/testpmd: add flow command parsing routine

testpmd flow creation is constructed from these procedures:
  1. receive string with flow rule description;
  2. parse input string and build flow parameters: port_id value,
     flow attributes, items array, actions array;
  3. create a flow rule from flow rule parameters.

Flow rule creation procedures are built as a pipeline. A new
procedure starts immediately after successful predecessor completion.
Due to this we have no dedicated routines providing intermediate
results for step 1-3 above.

The patch adds `flow_parse()` function call. It parses input string
and provides a caller with parsed data. This is a preparation step
for introducing flex item command processing.

Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Reviewed-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>