Change parameters of add_timestamp in ComputeDriverCPUMonitor class

Change parameter 'cls' to 'self' in wrapper. 'cls' may mislead developpers
that this decorator returns a classmethod. Use functools.wraps to follow
the common pattern of decorator in nova.

closes-bug: #1288281

Change-Id: I6acbcba7297607dc6d07e8668ad07e6ca81d4456
Merge "Change parameters of add_timestamp in ComputeDriverCPUMonitor class"