Override location of project environments

[mcg1969]

anaconda-project offers the environment variable ANACONDA_PROJECT_ENVS_PATH as a means to override the location where environments are created. By default of course they go in ./envs/<name>, but we may have reason to home the environments outside of the project itself.

This variable actually accepts a comma-separated list of values. New environments are created in the first writable directory on the path.

You can see how anaconda-project does this here:
https://github.com/Anaconda-Platform/anaconda-project/blob/master/anaconda_project/env_spec.py#L318-L348

More information here: https://anaconda-project.readthedocs.io/en/latest/config.html

[AlbertDeFusco]

PR #159 adds support to change the location of envs