Limit rabbitmq prefetch and use default rabbitmq

[stefpiatek]

Shouldn't need to increase the timeout for rabbitmq so reverting to pulling the image directly.

Testing these changes from the debug branch worked 🎉. Have started a new test run in /gae/pixl_dev to make sure that it doesn't disconnect overnight

Closes #143
Closes https://github.com/UCLH-Foundry/the-rolling-skeleton/issues/68

[jeremyestein]

Looks good, nice simple fix :)
It's worth noting that given our 2 minute processing timeout, this leaves unacked messages of max 20 minutes old, which is still below the rabbitmq ack timeout threshold of 30 minutes. However since we hit that 2 minute timeout quite often we may have to raise it, which might necessitate lowering the prefetch_count further.
I don't think we're gaining any notable performance from prefetching due to the long processing time for each message, so arguably 1 is a more robust value.

[stefpiatek]

Yeah that's a fair point, I'll set to 1 and we can readdress if we find a reason to change it