Load extra vagrant configuration from another file

Allow adding to the vagrant configuration with another file rather than
having lots of conditional code based on environment variables in the
`Vagrantfile`.

This loads the file specified in the POPIT_VAGRANT_EXTRAS environment
variable, which can contain another `Vagrant.configure` block which will add
to/override settings from the main `Vagrantfile`.

See https://docs.vagrantup.com/v2/vagrantfile/index.html for more
information (under the "LOAD ORDER AND MERGING" heading).