[soundwave] Code health in dashboard_api.py
[soundwave] Code health in dashboard_api.py

A few smaller things to improve code quality in this file:
- Factor out the common call to urllib.urlencode(params) from all
  _MakeApiRequest clients.
- Switch from 'benchmark' to 'test_suite' to be more consistent
  with terminology.
- Improve docs in a few places.

Bug: catapult:#4454
Change-Id: I9e682c69c3a87fc83c5049aecfc15147ab37e7b5
Reviewed-on: https://chromium-review.googlesource.com/1107932
Commit-Queue: Juan Antonio Navarro Pérez <1649bb5e6b9eafe026580e338065dc64ba6b80b2@chromium.org>
Reviewed-by: Charlie Andrews <532d7316e2689bea110898693dc3dc5671811b69@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult