PEP8: fix blank end of lines.

cobbler/action_buildiso.py:595:1: W391 blank line at end of file
cobbler/action_dlcontent.py:84:1: W391 blank line at end of file
cobbler/action_power.py:125:1: W391 blank line at end of file
cobbler/serializer.py:134:1: W391 blank line at end of file
cobbler/modules/serializer_couch.py:153:1: W391 blank line at end of file
cobbler/modules/serializer_file.py:198:1: W391 blank line at end of file
cobbler/modules/serializer_mongodb.py:161:1: W391 blank line at end of file