Prepare python3 environment

[YanChii]

The first step of full migration to python3 is to prepare the environment. This consist of

• creating new management VMs with already-installed python3
• install python3 in existing installations during upgrade

The second one is tricky because python is used also for upgrades (ansible). Therefore we have to install python before the appliance upgrade begins.

What is not covered by this issue:

• installing python3 in global zones and SunOS zones - this will be covered by planned pkgsrc archive replacement
• replace virtualenvs
• migrate esdc code to python3

[YanChii]

Python 3 cannot be used for ansible administration of CentOs 7 because it cannot control package installation with yum. Therefore we are staying on a stable supported path and python 3 will be used only where needed. This means we don't need to have python 3 installed before the upgrade starts and the task becomes easier.
It also means we will not merge the above PRs.