There are cases where we want to conditionally apply an
attribute to a test function, for example, if SSH validation
is enabled then a test may run much slower than if it is not.
This adds a 'condition' kwarg to the attr() decorator which
behaves similarly to the 'condition' kwarg on the skip_because()
decorator.
Change-Id: I83233854a217b6961e7614d7d9df1b4fc8d5a640