Poetry (https://python-poetry.org) is not declared as a dependency for a
Python project, it must be available somehow in the system. This role
installs it if missing.
- Latest version is installed, unless `ensure_poetry_version` is
informed.
- The installed executable path is set as the `poetry_executable` fact.
- The `/usr/local/bin/poetry` symlink can also be created if
`ensure_poetry_global_symlink: true`.
This new role is basically a copy of the `ensure-nox` role with the
symlink creation snippet taken from the `ensure-tox` role.
The commit adding `ensure-nox` (77b1b24) has been taken as an example of
the necessary changes when adding a new role.
Change-Id: I5592d38d415a9d74055348406653b69f110541ae
524b7e7b95