Fix pylint error in PapiExecutor

+ Remove ret_val as it cannot be nonzero outside _execute_papi.
+ Change indentation to save vertical space.
+ Return computed values without storing them to variable.
+ Add TODO to remove duplicated method.
+ Add missing fields to docstring.
+ Minor formatting changes.

Change-Id: I8e8f41d139de8aff3b76a7fe27ee450e89e8d1de
Signed-off-by: Vratko Polak <vrpolak@cisco.com>