- update Miniconda installer (required to install conda 4.4)
- pin conda package to defaults channel to avoid downgrade with
  conda-forge channel
- use new /opt/conda/etc/profile.d/conda.sh profile script (remove old
  conda_env.sh)

Note that conda base environment is not activated.

The wheel version that was used was removed (broken package?)

environment files shall be passed via the conda_env_files variable.

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.