Vagrantfile file checks

This request adds in a config item at the top of the vagrantfile which enables/disables openvpn, and some checks to warn the user if they do not have the appropriate files in the 'public' folder to enable a provision to work. If it is found that some files are missing, the user is prompted on whether to abort the command. Any answer other than 'no' or 'No' (after whitespace-stripping) will be interpreted as abort.

Hopefully this is moving towards vagrant being more user-friendly.

We may be able to extend the principle of the ELIFE_CONFIGURE options to include other similar variations.


Note: This request depends on https://github.com/elifesciences/elife-vagrant/pull/9 being merged first, I think