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
Fix security group list when not defined for an instance

If an instance has an empty security group list an exception
is thrown due to iteration attempt of None.

To fix this, this patch modifies get_instance_security_group
to return an empty list instead of None.

Change-Id: I7efb3c3a58d7e20501d126c6d194479ca1d14166
Closes-Bug: #1291489
Fix wrong method name for test_hacking

There were two methods with same name, let's fix it.

Change-Id: I114ff187e1128dd042ee836ac7b24fb5bcb9c265
Merge "Change parameters of add_timestamp in ComputeDriverCPUMonitor class"
Merge "Fix wrong method name for test_hacking"
Merge "Fix security group list when not defined for an instance"