updates linux platforms versions for test-kitchen

[vitalis]

• removes ubuntu 12.04 (End of life)
• adds ubuntu 16.04
• updates centos6 from 6.6 to 6.9
• updates centos7 from 7.1 to 7.3

[martinb3]

I was putting off doing this, thinking it wouldn't pass 👍 Huzzah.