pyenv_virtualenvs and pyenv_python_versions should be undefined

[00willo]

Thanks for developing this role, I've used it a great deal, appreciate your efforts.

I'd like to suggest the following changes to the way the pyenv_virtualenvs are handled.

1. Remove the pyenv_virtualenvs and pyenv_python_versions from defaults/main.yml
2. Add the when: pyenv_virtualenvs is defined to the Create virtual environments task.
3. Add the when: pyenv_python_versions is defined to the Create virtual environments task.

This way, if someone wants pyenv installed and no python versions or virtualenvs setup by default at the time this role is applied, then it handles that naturally. This allows the role to also be used to prepare an environment without requiring guesses to be made as to what the end use may need.