Migrate Ubuntu 16.04 LTS (Xenial) from py2 to py3

[arm4b]

Part of the StackStorm/community#53, StackStorm/st2docs#1033, StackStorm/community#40.

Similar to EL7 py2->py3 transition #671, switch Ubuntu Xenial 16.04 LTS to python 3.

[arm4b]

U16 + py3 deb packaging is done via #679

Keeping this issue open as there's a follow-up needs to be done for curl|bash scripts:
#679 (comment)

Based on @StackStorm/tsc discussion, the solution is to pause the curl|bash st2 installation waiting for the user input when Ubuntu16.04 has no available py3 package candidate.

• yes will install the Deadsnakes PPA, no will exit the installation process.
• there should be a Warning that users are about to add 3rd party py3 PPA on their own security risk and recommend to migrate to U18.
• We'll need a CLI flag --maybe-yes (naming TBD) to allow automatic installs.
• Same for Ansible/Puppet install.
• Ubuntu 16.04 py2 -> py3 upgrades, - users have to rely on a manual Upgrade instructions we'll provide in st2docs. Recommend 3rd party Deadsnakes PPA with a security disclaimer.

That'll give us acceptable balance between security/usability and we could e2e and CI things as well.

we'll also need st2ci and st2docs update to reflect the described changes.