Allow multiple environment path directories

[mcg1969]

This PR modifies the interpretation of ANACONDA_PROJECT_ENVS_PATH so that multiple environment directories can be considered. Rules:

• The variable is assumed to be a list of paths, separated by os.pathsep
• If an entry is empty, it is replaced with envs
• Relative paths are considered relative to the project directory (which is why envs is the appropriate default)
• If an environment with the given name does not exist anywhere on the path list, it is created in the first directory.
• If the directory is outside of the project directory, it is not removed by cleanup commands.
  So for example, suppose you want the default directory to still be the envs subdirectory, but you want to allow the child environments in your local Miniconda installation ~/miniconda3 to be considered. Then the value

export ANACONDA_PROJECT_ENVS_PATH=:~/miniconda3/envs

will produce the desired results.

[mcg1969]

The one test failure so far is a timeout on the GA side

[AlbertDeFusco]

Very cool.

[mcg1969]

Thanks... I'm very close to having persistent sessions work and this is essential for that