bifrost: fix creation of python3 lib symlink

When {{ venv_path }}/lib/python3 already exist and we run the ln
command, we end up with:

    /var/lib/kolla/venv/lib/
    ├── python3 -> python3.9
    ├── python3.12
    │   └── site-packages
    └── python3.9
        ├── python3.12 -> python3.12
        └── site-packages

Instead, it should be:

    /var/lib/kolla/venv/lib/
    ├── python3 -> python3.12
    ├── python3.12
    │   └── site-packages
    └── python3.9
        └── site-packages

Remove the existing link first to ensure it is created properly.

Change-Id: I42dd2490d14070074fe1fa7ae9e0a4cbf095b06f

Author: priteau

docker/bifrost/bifrost-base/Dockerfile.j2

@@ -48,7 +48,8 @@ RUN mkdir -p /requirements \
     && python3 -m venv --system-site-packages {{ venv_path }} \
     && KOLLA_DISTRO_PYTHON_VERSION=$(/usr/bin/python3 -c "import sys; print('{}.{}'.format(sys.version_info.major, sys.version_info.minor))") \
     && cd {{ venv_path }}/lib \
-    && ln -s python${KOLLA_DISTRO_PYTHON_VERSION} {{ venv_path }}/lib/python3
+    && rm -f python3 \
+    && ln -s python${KOLLA_DISTRO_PYTHON_VERSION} python3
 
 RUN ln -s bifrost-base-source/* bifrost \
     && {{ macros.install_pip(['pip', 'wheel', 'setuptools']) }} \