CFY-650 enabled on_failure override for tasks
CFY-650 added doc comments for Task's on_success and on_failure handlers parameters
Merge pull request #9 from cloudify-cosmo/feature/CFY-650-implement-default-uninstall-workflow

CFY-650 enabled on_failure override for tasks
CFY-650 flake8 fixes
Merge branch 'develop' into feature/CFY-718-remove-dependency-links