Add timer for atomic actions

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