Httpchallenge agent plugin config improvement.

[edwbuck]

The Httpchallenge agent plugin opens a temporary web server to receive the response of the httpchallenge server plugin. This temporary web server can be launched in one of two configurations:

1. Normal operation - the web server binds to all hostnames.
2. Unit testing operation - the web server binds to a configurable hostname.

The Unit testing operation was added specifically due to the need to avoid all hostnames in our build system under CircleCI, where we override the listening port to bind to localhost.

A few small opportunities for improvement exist:

• Promoting the ability to bind to a specific hostname to the plugin's runtime configuration
• Replacing the "side door" logic for overriding the listening hostname in unit tests with the feature to specify it in the configuration suggested here.