dpdkstat plugin rework

dpdkstat plugin rework

This patch contains rework of existing dpdkstat plugin. Changes are focused on
extracting common dpdk related features into utils file that could be reused by
other DPDK plugins. Also some related structural changes are introduced.

List of changes:
- introduction of new utils_dpdk.c that handles child process spawning,
  and terminating, messaging between parent and child, DPDK configuration
  and initialization and DPDK commands execution
- DPDK EAL specific options put into separate subsection of
  dpdkstat configuration
- documentation update

Those changes are preparatory steps for upstreaming dpdkevent plugin
that is going to collect DPDK ports and logical cores status info.

Signed-off-by: Krzysztof Matczak <krzysztofx.matczak@intel.com>