Adjust the proxy_user setting for the curl downloader to proxy_username.

In order to take advantage of the new proxy support in the downloader, we needed to make a tiny change in the ISO importer to call a setting proxy_username instead of proxy_user. It's a very simple fix, and needs to be merged to 2.1 and master after review.

This pull request depends on https://github.com/pulp/pulp/pull/426 so please review that one first.