Provide Python 3 interpreter as python3 executable

Summary:
On FreeBSD, in scenarri where the metaport python3 is missing,
and only python3xx packages are installed, the python3 symlink can be absent.

As our scripts generally use #!/usr/bin/env python3 and not a precise shebang
targetting a specific version of Python 3, python3 should exist and be linked
to the version used by system dependencies.

This fix picks the lower version, as packages tend to be shown in that order.

Test Plan: salt -G 'os:freebsd' state.apply hotfixes/python3

Reviewers: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3288