Introduce replay_end_time load test parameter to compliment replay_start_time

[ysawa0]

Currently the replay time window for a load test is specified using "replay_start_time" and "loop_duration"
This feature will allow a user to specify replay time window like so:

test_params: {
"replay_start_time": "2018-08-06T13:00",
"replay_end_time": "2018-08-06T14:00"
}

instead of:

test_params: {
"replay_start_time": "2018-08-06T13:00",
"loop_duration": 60
}