Does it make sense for a policy file to have multiple urls?

[geky]

From #53 (comment), does it make sense for a policy file to contain multiple sinaloa_urls? Multiple tabasco_urls (maybe for falbacks if a server is down?)?

This affects both the policy structure (multiple urls, multiple enclave hashes), as well as how we structure the internal code (can you instantiate multiple Mexican cities in the same binary).