voltha system test: extend test suite by further abort tests (see description)

1. abort when downloading (already done with VOL-4318)
2. abort when downloaded (but not yet activated)
3. abort when activating (already done with VOL-4319)
4. abort when active but commit not set
5. abort when committed by checking BBSIM server transfer image counter after further download, counter should be incremented
6. checking BBSIM server transfer image counter by downloading same image after already active and committed but without aborting, counter should be not incremented
7. downloading image to all ONUs simultaneously (Multiple OLT scenario)