Switch Docker images from Ubuntu Xenial -> Bionic as a base #74
Description
Per https://docs.stackstorm.com/roadmap.html starting from st2 v3.1 Bionic will be going from beta to GA state with Python 3.6 used by default.
We'll need to switch our Dockerfiles to latest Bionic as it's something that's needed for internal infra/py3 testing efforts and would be good in general for our community as well.
Complementing PR for Dockerfiles: StackStorm/st2-dockerfiles#16