Dynamic worker url configuration

[chrisnatali]

When bringing a server up and down on ec2, the ip address changes. Since the worker url is currently set statically in a config, this requires a manual modification for each down/up of worker. This is a barrier to dynamic up/down/resize of workers on ec2.

We can derive the public ip and set the url dynamically (though assumes external service is up):
https://stackoverflow.com/questions/2311510/getting-a-machines-external-ip-address/36205547#36205547

Or we could expose a worker_init endpoint in primary which returns the public ip back to the worker using something like:
https://stackoverflow.com/a/28959670/2044042

This code needs to change