Isolate dependencies for generate-devstack-plugins-list.sh by
creating a dedicated Python virtual environment with required
packages. This ensures the script uses the correct version of
dependencies regardless of system Python configuration.
Closes-Bug: #2138869
Change-Id: I8a7e49b0b908aec7c95b7fbfb6c9a16375f267a6
Signed-off-by: Abhishek Bongale <abhishekbongale@outlook.com>