Add a Vagrantfile to use the install script in a Vagrant box

This should be used with:

  vagrant --no-color up

... which (after some time) will leave you with a running site at:

  http://alaveteli.10.10.10.30.xip.io

Many thanks to Simon Coffey (@urbanautomaton) and Chris Adams
(@mrchrisadams) who produced a working Vagrantfile with a
independent install script while our installation script work
was ongoing - their advice and that script were very helpful
for creating this Vagrantfile that uses our generic mySociety
installation script.
Set 'constraint_disabling: false' for tests in database.yml

Without this setting, the PostgreSQL user still has to be a superuser
for the tests to work without constraint disabling errors.