Ensure right conda version is used

[nchaimov]

If a Python 3 Anaconda is present in the environment, TAU Commander configuration will attempt to use it instead of downloading Miniconda.

For the current Python 2 version, we should not attempt to use a Python 3 conda found in the environment.

When ported to Python 3, we should have checks to ensure that we don't try to use a Python 2 anaconda from the environment or from an old TAU Commander install.