Clean up formatting in PKB file - working on aerospike_benchmark.py file

Hello
I am working on the this issue in aerospike_benchmark.py file

Goal to convert 
flags.DEFINE_integer(
    'aerospike_client_threads_step_size',.....)

to 

AEROSPIKE_CLIENT_THREAD_STEP_SIZE = flags.DEFINE_integer(
    'aerospike_client_threads_step_size',.....)

and change its uses accordingly