vagrant: Force re-install of existing folders

When "vagrant provision" is run, it uses "setup.py install" to install the latest development branch of Plinth in the vagrant box. However, "setup.py install" doesn't install over existing folders such as /usr/share/plinth, which contains actions and static files. This can lead to missing files when trying to test Plinth.

Investigate if we can force setup.py to overwrite the existing files.