make filter pod by benchmark optional

[sunya-ch]

To avoid misunderstanding from validate function, I make the function to filter the pods by inputting benchmark name to be optional with recommendation in the README file.

Result with filter:

===========================================
 Use benchmark name to filter pod results:

 stressng
                                                               count   >0
scenarioID          query
                    kepler_container_bpf_block_irq_total         527    0
                    kepler_container_bpf_cpu_time_ms_total       527  260
                    kepler_container_bpf_net_rx_irq_total        527  121
                    kepler_container_bpf_net_tx_irq_total        527   58
                    kepler_container_bpf_page_cache_hit_total    527    0

Result without filter:

============================================
 Present results from all pods:

                                                               count    >0
scenarioID          query
                    kepler_container_bpf_block_irq_total        4046     0
                    kepler_container_bpf_cpu_time_ms_total      4046  3374
                    kepler_container_bpf_net_rx_irq_total       4046  2956
                    kepler_container_bpf_net_tx_irq_total       4046   193
                    kepler_container_bpf_page_cache_hit_total   4046     0

Signed-off-by: Sunyanan Choochotkaew sunyanan.choochotkaew1@ibm.com