app/testpmd: tune default parameters

app/testpmd: tune default parameters

We usually use testpmd fwd to demonstrate IO forwarding throughput.
For best throughput, it has to assign special parameters to testpmd.
To make it easier to run, now set it as defalut value.
Such parameters are MBUF Mempool Cache and RX/TX threshold registers.
    MBCACHE: 250
    RX threshold registers: pthresh=8 hthresh=8 wthresh=0
    TX threshold registers: pthresh=32 hthresh=0 wthresh=0

Signed-off-by: Cunming Liang <cunming.liang@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Yong Liu <yong.liu@intel.com>
Tested-by: Zhaochen Zhan <zhaochen.zhan@intel.com>