This patch:
1. Resolves an issue where the base image was overrided with python:3.6
2. Adds framework for creating new images of other distributions and
gating against them.
Change-Id: Iaa8ec86f947189bf24d40e5594e015554f7abddb
Per community discussion updating default image to ubuntu:16.04 in
place of python:3.6 for two reasons:
1. Reduce number of vulnerabilities in the built image
2. Standardize across Airship the default image used as ubuntu:16.04
Change-Id: I598989ae15a86520850dbf6693feab659a135fff