Support Symfony initial builds with a seed database

[gregharvey]

The ultimate purposes of this is to allow Symfony projects to have feature branching.

At the moment we can only seed databases with WordPress and Drupal. Symfony presents a challenge, as by its nature the apps tend to be bespoke, so we can't know what the parameters.yml.dist attributes for the db name, username and password will be in advance, nor can we create them and safely add them to the parameters.yml include we generate.

I think, in order to achieve this, we'll need to use the database generation code (which is already in common/MySQL.py) and then have some clever way of allowing developers to tell us which attributes are to be used for the different generated database attributes, which we'll then have to pipe into the correct attributes in the generated parameters.yml file in /var/www/config - it won't be trivial!

Be a good thing to sort out though, if we do it right it'll probably help us further abstract database handling, which is only a good thing.