Specialise the web proxy role ahead of adding new roles

[PeterJCLaw]

Summary

We're going to use this ansible config for more than just the web proxy host, which means we need to specialise the roles which aren't common to all machines. This commit does this both for ansible and Vagrant, so that our Vagrantfile can work for other machines as well.

Code review

Testing

• [x] applied the configuration locally
• [x] manually validated the new behaviour