Rewrite makefile to more easily support building other distributions.

At MoJ we are currently stuck using CentOS (largely for historical reasons and all our tooling is CentOS based right now) but we don't really have a clean portable way of building golden images for vCloud, Vagrant and EC2.

This pull request tidies up the make process to make building other appliances nicer - you don't need to add any new make rules to build from a .appl file to a finished (VMWare) .ova. You can now do:

```
make ubuntu-precise
```

Will look for ubuntu-precise.appl and build the .ova including dealing with differing filenames and `name:` directives (which bit me by mistake)

I will plan on adding support for being able to build for different providers (namely EC2 and possibly vagrant).

I've used this PR to build a Centos6.5 VM (using the Fedora 15 meta-appliance from boxgrinder.org) and the ubuntu-precise appliance using the ubuntu-precise-boxgrinder vagrant box.