Add dpdk per-interface startup config parameter to specify worker threads

Add dpdk per-interface startup config parameter to specify worker threads

New parameter allows specifying which worker threads will process rx
queues. Parameter arguments is list of cores and number of worker specified
must be equal to the number of rx queues configured (num-rx-queues). If
num-rx-queues is not specified, it will be automatically set to
number of workers.

Sample config:

dpdk {
  dev 0000:86:00.0 {
    workers 2,3
    num-rx-queues 2
  }
}

Change-Id: I88bc381e0e542eb02def09a726c6f04de3e1ae17
Signed-off-by: Damjan Marion <damarion@cisco.com>