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"
Merge pull request #191 from alphagov/govuk_seed_crawler_amqp

Create RabbitMQ user for govuk_seed_crawler