Move cleanup functions to benchmark.utils

This is an initial effort to cleanup ScenarioRunner before it is
refactored to sparate scenario types.

Change-Id: Icd2002b3a8919bcbb162e5a75e17f00891b04199
Trello: https://trello.com/c/JN6xUY79
Validator for image_id

Add validator that checks image with specified image_id exists.

Change-Id: I1ea83ae18f5b365ebe8d6da765b544138aebdac4
Merge "Move cleanup functions to benchmark.utils"
Merge "Validator for image_id"