Upgrade to Debian 9 stretch

[seanho00]

• [x] Make a dist-upgrade play+task, like what I have for regular upgrades
• [x] Test: KVM
• [x] Review package list inv/groups/all/apt.yml
• [x] Test: OVZ
• [x] Test starting from clean Deb8 (reinstall):
  • standard plays/setup.yml
  • upgrade to 9: plays/dist-upgrade.yml
  • standard plays/main.yml
  • setup.yml and main.yml should work on either clean 9, or clean 8 upgraded to 9
• [x] Rollout to other hosts
• [x] Migrate VPN host
• [x] Migrate dedi