Miniconda2 was made the default because of Ansible, but there is no
reason.
Ansible has to use the default Python installation in the first place.

For molecule, there is no problem to install that in a conda env with
Python2.

A new miniconda installer is not released with every conda version.
`conda_version` is now independant of `miniconda_version`.

Add tests on conda user

Ansible still requires Python 2.
We make Miniconda2 the default.

It's still possible to install Miniconda3 by overriding
miniconda_version and miniconda_installer_md5.

sudo is required to install conda.
It's automatically installed by molecule so the missing dependency was
not detected by molecule test.