zdtm: Replace imp module with importlib

imp.load_source() has been deprecated [1]. The recommended alternative
API for loading a module is exec_module() [2].

[1] https://docs.python.org/2.7/library/imp.html#imp.load_module
[2] https://docs.python.org/3.4/library/importlib.html#importlib.abc.Loader.exec_module



Signed-off-by: Radostin Stoyanov <rstoyanov1@gmail.com>