Allow to specify the channels to use using the conda_channels variable.

1. It's easier to use a mirror (no need to manually copy new RPMs to
artifactory)
2. The mirror is configured in this role because it's the only repo needed to
install conda (no EPEL). No need to put it in the ics-ans-role-repository and
add a dependency.

RPM released by anaconda.
Installation is done from the RPM to avoid adding the repository as
dependency.

Still issue with the new installer:
NotWritableError: The current user does not have write permissions to a required path.\n  path: /home/conda/.conda/pkgs/urls.txt

Update installer as well

Installer NOT updated to 4.4.10 due to this bug:
https://github.com/conda/conda/issues/7109

Have to wait for a newer installer before to update.
The installer doesn't really matter as long as we can update conda to
the version we want.

- 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.

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.