client option to force use of single address for Pilosa

[jaffee]

Description

This is a client issue, but it applies to all clients, so I've made it in the Pilosa repo.

In the situation where Pilosa is running behind a proxy or load balancer, the client needs to always use the proxy/load balancer address, and not an address that it gets from Pilosa's /status endpoint.

I think this is only needed for imports, but we should make sure.

Relates to #1759

Success criteria (What criteria will consider this ticket closeable?)

Pilosa clients have an option to force use of given address always rather than taking addresses from Pilosa.