Update the bootstrap-ansible script to install equivalent Python3 distro
packages on Ubuntu and default to Python3 when creating the Ansible
virtualenv. If an existing virtulenv exists with a differing version of
Python, fully remove it for a cleaner installation.
Change-Id: I06cf934b3af1005d4b2adec76e22f249b72a69ec
Implements: blueprint goal-python35
Depends-On: I8874c439fc9c006125bb39ff6a2cd386bfc8f561
Depends-On: I7270eef4b03fdd4cdf8b578edb479ecb336b6a83
Depends-On: If2484febbab4db20539749094517240b5f17d1ab