Create RabbitMQ user for govuk_seed_crawler

This will allow it to create exchanges and queues that are prefixed with
"govuk_seed_crawler". In my testing and I haven't found that it
creates/needs any automatically generated resources of the format "amq.gen"
I don't think this is correct. It'll need to write to the exchange at `govuk_crawler_exchange` when running this for real. You probably want this to be `^(govuk_seed.*|govuk_crawler_exchange)$`.