Minor refactor of add_labels_in_df
Minor refactor of add_labels_in_df

CLOUDBLD-8123

- never write to self.labels outside init, instead return new labels
  from methods and add them to a local copy of the labels
- fix import order
- various minor simplifications

Signed-off-by: Adam Cmiel <1217f9865bd733d1bcad0d0d64be310272c5592c@redhat.com>

fr34k8/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor,projectatomic/atomic-reactor