vagrant: Vagrantfile changes for ease of development

Moved some frequently required operations to the Vagrantfile.

Newcomers to the project almost always face the issue of port 8000 not being available. Disabling system Plinth and running only the development version solves this issue.

Automatic upgrades are a frequent blocker for development and an unnecessary annoyance on a development setup.

Signed-off-by: Joseph Nuthalapati njoseph@thoughtworks.com