Execution time is now measured for atomic actions, so we have time
measuring for the whole benchmark and also detailed for each atomic
action.
Added the decorator atomic_action_timer and used it in nova, cinder
and keystone scenarios.
Partially implements: blueprint time-measuring-of-atomic-actions
(Part "I) Core" of time-measuring-of-atomic-actions)
Change-Id: I99571ef1221757784ce2b85889ad989fcae05bb2