replace --hatch-rate and --startup-time with --increase-rate, --increase-time, --decrease-rate and --decrease-time

[jeremyandrews]

Both --hatch-rate and --startup-time assume that we are only configuring a load test to ramp up. Now that load tests are internally represented with a test plan, we also have to take in the a ramp down time.

The existing commands would be replaced by the --increase commands.

To configure a ramp-down time, we'd introduce the new --decrease commands.

Beyond building load test with a ramp-up time and/or a ramp-down time, this can also be useful when adjusting a running load test through the controller once #454 lands.