Check for connection to remove only in the parent PG
Keep track of which controllers have been removed to avoid removing them twice (creating a set would have been an alternative)
Remove only controllers inside the process group
Coverage decreased (-0.3%) to 66.667% when pulling 11822c1170e7f4d7f7d89fc6feea55f074495fc5 on rsaggino:fix-force-pg-delete into b5297c2d825ba47be729e7906292f44ae9f9f642 on Chaffelson:master.
Fixing #167,#180
Tested on NiFi 1.9 with Python 3.6