Don't use an http proxy when connecting to localhost

We had a PE site which had set the http_proxy_{port,host} settings for PMT usage, which had the unintended consequence that all puppet master http connections went through the proxy, including those to the dashboard for node lookups. When something (not yet identified) changed in the network/proxy configuration, these lookups failed causing all agent runs to fail.

This raises the question about whether it ever makes sense for the master to use a proxy for connections to localhost.

We've considered two approaches here:
1) add a new setting for http proxy exclusions. This could be set to, e.g. localhost;*.internal.lan, etc.
2) never using the proxy settings when talking to localhost

There may be some other approaches that would work as well.